This application is a Division of application Ser. No. 08/768,586 filed Dec. 18, 1996 which is a continuation of application Ser. 08/297,918 filed Aug. 31, 1994 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an image recording method and apparatus. For example, the invention relates to control of an optional unit which extends the paper handling function of an image recording apparatus for recording images using an electrophotographic process. 
     An image recording apparatus such as a laser printer using an electrophotographic process is constructed as shown in FIG.  1 . The apparatus has a main body  201  which includes a video controller  202  and an engine controller  203 , below. 
     The video controller  202  mainly is for performing control related to image processing. Specifically, the video controller  202  develops image information into bit data. The image information is sent from an external device (not shown), such as a personal computer, via a general-purpose interface such as a Centronics interface. The engine controller  203  mainly is for controlling corona discharge, exposure, development, transfer, fixing and paper conveyance related to the electrophotographic process. A video interface  204  interconnects the video controller  202  and engine controller  203  to make possible communication between them. 
     A paper deck option  205  accommodates a large supply of recording paper and supplies the recording paper to the image recording apparatus  201  in response to an instruction from the engine controller  203 . A paper deck interface  206  interconnects the engine controller  203  and paper deck option  205  to make possible communication between them. 
     A sorter option  207  has a plurality of stacking trays for stacking sheets of recording paper, which have been recorded on and discharged from the image recording apparatus  201 , while assorting them into prescribed categories in response to an instruction from the engine controller  203 . A sorter interface  208  interconnects the engine controller  203  and sorter option  207  to make possible communication between them. 
     In the arrangement described above, the video controller  202  accepts image information and a print instruction from the external device, whereupon the controller  202  sends the engine controller  203  a video signal, obtained by converting the image information into bit data, as well as the print instruction via the video interface  204 . Further, on the basis of a designation made by the user from an operation panel (not shown) connected to the external device or video controller  202 , the video controller  202  also transmits a command relating to use of the paper deck option  205  or sorter option  207 . 
     The engine controller  203  controls the components related to tho electrophotographic process, namely the components for corona discharge, exposure, development, transfer, fixing and paper conveyance, within the image recording apparatus  201  and carries out image recording on the basis of the received video signal. At the same time, on the basis of a command from the video controller  202 , the engine controller  203  controls the paper deck option  205  via the paper deck interface  206  to designate the timing of paper feed, and controls the sorter option via the sorter interface  208  to designate assorting of the sheets of recording paper. 
     Various problems are encountered in the art described above. 
     Specifically, the engine controller  203  is connected to the two optional units  205 ,  207  in a fixed relationship and this combination cannot be changed. Accordingly, in order to realize control of a new optional unit, the engine controller is additionally provided with a control capability other than that for controlling the electrophotographic process. Further, in order to provide higher functionality, it is necessary to improve the capabilities of the engine controller which will control these functions. 
     Environments in which image recording apparatus are used have become more diverse in recent years and this diversification has been accompanied by a desire for higher functionality of installed optional units and for the ability to use a wide variety of optional units freely in different combinations. However, in order to make it possible to control such a wide variety of optional units in the arrangement described above, it is necessary to improve the capabilities of the engine controller and add on controlled systems. The result is a rise in the cost of the engine controller. This rise in cost is a meaningless price increase for users who do not intend to employ optional units. 
     Furthermore, in a case where a plurality of optional units are combined, it is necessary to know the order in which each unit is connected. In addition, since the recording paper is conveyed between optional units at high speed, it is required that instructions for control of paper conveyance be sent to the plurality of optional units simultaneously and at high speed. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an image recording apparatus provided with a master controller, in addition to the video controller and engine controller, for overall control of a variety of optional units, means whereby the video controller applies instructions directly to the master controller, and means for notifying the video controller and master controller of timing signals from the engine controller relating to paper discharge and paper feed, thereby making it possible to use a variety of optional units freely in any combination without restrictions being imposed by the functions of the engine controller. 
     According to the present invention, the foregoing object is attained by providing an image recording apparatus comprising control means for generating an image signal from image information based upon a recording instruction, recording means for recording an image based upon the image signal generated by the control means, recording control means for controlling the recording means, and optional-unit control means for controlling at least one optional unit, wherein the control means applies control instructions to the optional-unit control means, without intervention of the recording means, in response to the recording instruction. 
     Another object of the present invention is to cause means which controls an optional unit to control conveyance of recording paper, thereby freeing the image recording apparatus from control of conveyance of recording paper to a plurality of optional units. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating the construction of an image recording apparatus such as a laser printer using an electrophotographic process; 
     FIG. 2 is a diagram showing the construction of an embodiment of an image recording apparatus according to the present invention; 
     FIG. 3 is a block diagram showing the manner in which each controller in FIG. 2 is connected; 
     FIG. 4 is a block diagram showing the detailed connections between a master controller in FIG.  2  and the controller of each optional unit; 
     FIG. 5A is a timing chart showing the manner in which data is transferred in FIG. 4; 
     FIG. 5B is a timing chart showing another way of applying strobe signals in FIG. 4; 
     FIG. 6 is a flowchart showing a procedure for verifying connection of optional units; 
     FIG. 7 is a block diagram showing another manner of connecting each controller of FIG. 2 in a second modification; 
     FIG. 8 is a block diagram showing another method of connecting a master controller and the controller of each optional unit in a third modification; 
     FIG. 9 is a diagram showing the construction of an image recording apparatus according to a fourth modification; 
     FIG. 10 is a block diagram showing the manner in which each controller in FIG. 9 is connected; 
     FIG. 11 is a diagram showing the construction of an image recording apparatus according to a fifth modification; 
     FIG. 12 is a block diagram showing the manner in which each controller in FIG. 11 is connected; 
     FIG. 13 is a diagram showing the construction of an image recording apparatus according to a sixth modification; and 
     FIG. 14 is a block diagram showing the manner in which each controller in FIG. 13 is connected. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of an image recording apparatus according to the present invention will now be described in detail with reference to the drawings. 
     FIG. 2 is a diagram showing the construction of an image recording apparatus according to the present invention. A laser-beam printer is illustrated in this embodiment by way of example. In the example illustrated, two optional units are connected, though it is possible to connect a large number thereof. Construction and operation will now be described. 
     Construction 
     In FIG. 2, numeral  1  denotes the main body of a laser-beam printer in which recording paper S accommodated in a recording paper cassette  2  is supplied to the body  1  by a paper feed roller  3 . A pair of resistance rollers  5  for synchronously conveying the recording paper are provided downstream of the feed roller  3 , and an image recording section  7  for forming a toner image on the recording paper by laser light from a laser scanner  6  is provided downstream of the resistance rollers  5 . 
     Further, a fixing unit  8  for thermally fixing the toner image formed on the recording paper is provided downstream of the image recording section  7 . Provided downstream of the fixing unit  8  are a paper-discharge sensor  9  for sensing the state of recording paper conveyance in a paper discharge section, conveyance rollers  10  for conveying the recording paper, and a flapper  11  for switching the direction in which recording paper, on which recorded has been completed, is conveyed. By switching the flapper  11 , the recording paper is stacked on a stacking tray  13  through paper discharge rollers  12  or conveyed to optional paper discharge rollers  14  in order to be conveyed to a paper discharge option. 
     The laser scanner  6  includes a laser unit  15  for emitting laser light modulated based upon an image signal VDO sent from a video controller  26 , described later, a motor  16  for driving a polygon mirror which causes the laser beam from the laser unit  15  to scan a photosensitive drum  20 , a beam detector  17  for detecting the position of the laser beam in the scanning direction, an imaging lens group  18  and a folded mirror  19 . 
     The image recording section  7  includes elements necessary for a well-known electrophotographic process, namely the photosensitive drum  20 , a pre-exposure lamp  21 , a primary corona discharge device  22 , a developing unit  23 , a transfer corona discharge device  24  and a cleaner  25 . 
     An engine controller  26  controls the electrophotographic process involving the laser scanning section  6 , image recording section  7  and fixing unit  8 , and controls the conveyance of recording paper within the main body  1 . A video controller  27 , which is connected to an external device (not shown) such as a personal computer by a general-purpose interface (Centronics or RS232C), develops image information sent via the general-purpose interface into bit data and sends the bit data to the engine controller  26  via the signal VDO. A video interface  28  makes possible communication between the video controller  27  and the engine controller  26 . 
     There are cases in which the recording means including the above-described laser scanning section  6 , image recording section  7 , fixing unit  8  and the recording-paper conveyance path (hereinafter referred to as the “paper path”) from the resistance rollers  5  to the paper discharge rollers  12  or optional paper discharge rollers  14  is referred to as a printer engine. Further, there are cases in which the paper path from the resistance rollers  5  to the conveyance rollers  10  is referred to as a common conveyance path. 
     Optional Units 
     Various optional units capable of being detachably connected to the main body  1  will now be described. 
     An optional master unit  32  has a master controller  101  which exercises overall control of a variety of optional units. The master controller  101  is connected to the video controller  27  via a master interface  114  connected via a connector  31 . The optional master unit  32  has feed-interrupting conveyance rollers  33  for interrupting conveyance of the recording paper from an optional unit in the paper feed system, a feed-interrupt conveyance path  34 , optional paper feed rollers  4  for supplying the interior of the main body  1  with recording paper conveyed by the feed-interrupt conveyance path  34 , and discharge-interrupting conveyance rollers  35  for interrupting conveyance of recording paper to an optional unit in the paper discharge system and a discharge-interrupt conveyance path  36 . 
     A large-capacity paper deck optional unit  40  accommodating recording paper has a paper deck  41  which is raised and lowered. Sheets of recording paper stacked on the deck are supplied using a paper-deck feed roller group  42  as necessary. The optional unit  40  includes feed-interrupting conveyance rollers  38  for interrupting conveyance of the recording paper from another optional unit in the paper feed system, a feed-interrupt conveyance path  44 , discharge-interrupting conveyance rollers  45  for interrupting conveyance of recording paper to an optional unit in the paper discharge system, and a discharge-interrupt conveyance path  46 . The paper deck optional unit  40  is controlled by a paper-deck controller  102 . 
     A sorter optional unit  50 , which has a plurality of paper discharging bins, a first bin  51 , a second bin  52 , a third bin  53  and a fourth bin  54 , assorts and stacks sheets of recording paper that have already been recorded on. The assorting of recording paper in each of the bins is performed by first, second, third and fourth flappers  55 ,  56 ,  57  and  58 , respectively. The optional unit  50  includes feed-interrupting conveyance rollers  61  for interrupting conveyance of the recording paper from an optional unit in the paper feed system, a feed-interrupt conveyance path  62 , discharge-interrupting conveyance rollers  60  for interrupting conveyance of recording paper to another optional unit in the paper discharge system, and a discharge-interrupt conveyance path  59 . The sorter optional unit  50  is controlled by a sorter controller  103 . 
     The master controller  101 , paper-deck controller  102  and sorter controller  103  are connected by optional-unit interfaces  70  interconnected by connectors  71   a,    71   b  and connectors  72   a,    72   b.  Numerals  71   a,    72   a,    73   a  denote identical connectors, and numerals  71   b,    72   b  represent identical connectors. Accordingly, the paper-deck optional unit  40  and sorter optional unit  50  can be interchanged in terms of position. 
     FIG. 3 is a block diagram showing the manner in which each controller is connected. 
     The master interface  114  is constituted by a serial communication interface  80  and four hardware signals, namely an OPTRDY signal  81 , a POUTT signal  82 , a PFEDT signal  83  and an SPCNG signal  84 . 
     Commands such as for designating paper feed to the paper-deck optional unit  40  and designating discharge bins for the sorter optional unit  50  are transmitted from the video controller  27  via the serial communication interface  80 . Further, various states such as whether or not there is recording paper in the paper-deck optional unit  40 , the stacking of paper in each discharge bin of the sorter optional unit  50 , etc., are transmitted from the master controller  101  via the serial communication interface  80 . It should be noted that the master controller  101  and the video controller  27  can coupled directly by a CPU bus. 
     The OPTRDY signal  81  indicates whether an option designated by the video controller  27  is in a usable state or not. The POUTT signal  82  is a timing signal indicating the timing at which the main body  1  discharges recording paper. The PFEDT signal  83  is a timing signal indicating the timing at which the main body  1  accepts recording paper from an optional unit. The SPCNG signal  84  is a signal for slowing down the speed of the recording paper, which is conveyed at high speed through the interior of an optional unit, so as to match this speed to the conveyance speed of the main body  1 . The three signals POUTT  82 , PFEDT  83  and SPCNG  84  are outputted by the engine controller  26 , pass through the video controller  27  via the video interface  28  and enter the master controller  101 . 
     Operation 
     In the arrangement shown in FIGS. 2 and 3, the master controller  101  sends the paper-deck controller  102  a command, via the optional-unit interface  70 , requesting information status relating to the length of the paper path and the conveyance speed of the paper-deck optional unit  40 . The master controller  101  then obtains this information. Further, the master controller  101  sends the sorter controller  103  a similar command to obtain information relating to the length of the paper path and the conveyance speed of the sorter optional unit  50 . It should be noted that these items of information have been stored in a non-volatile memory within each controller. 
     On the basis of the information thus obtained, the master controller  101  calculates the recording-paper delivery timing of each optional unit and adjusts the timing at which the conveyance rollers of each optional unit are turned on and off. 
     In a case where a print instruction for feeding paper from the paper-deck optional unit  40  and discharging paper into the sorter optional unit  50  has been issued by the external device (not shown) such as a host computer, the video controller  27  responds to this instruction by instructing the master controller  101 , via the serial communication interface, to supply paper from the paper-deck optional unit  40 . 
     Upon receiving this instruction, the master controller  101  instructs the paper-deck controller  102 , via the optional-unit interface  70 , to pick up and convey recording paper. The recording paper is supplied as a result. Further, the master controller  101  instructs the paper-deck controller  102  to halt conveyance at a timing at which the leading edge of the recording paper arrives at the optional paper feed rollers  4 . This timing is calculated from the information relating to the length of the paper path and conveyance speed of the paper-deck optional unit  40 . 
     During this series of operations for supplying paper, the video controller  27  instructs the engine controller  26 , via the video interface  28 , to begin printing. 
     Upon receiving this instruction, the engine controller  26  starts the printer engine and then raises the PFEDT signal  83  to logical “1” when the recording paper has been accepted by the main body  1 . When the PFEDT signal  83  rises to logical “1”,  the master controller  101  instructs the paper-deck controller  102 , via the optional-unit interface  70 , to resume the conveyance of the recording paper. 
     As a result, the recording paper is advanced forwardly of the optional paper feed rollers  4 . When the recording paper is sensed by a sensor  30 , the engine controller  26  raises the SPCNG signal  84  to logical “1”. When this is done, the master controller  101  instructs the paper-deck controller  102 , via the optional-unit interface  70 , to change the conveyance speed of the recording paper in order to make the conveyance speed of the recording paper in the paper-deck optional unit  40  agree with the conveyance speed of the recording paper in the main body  1 . The reason for this is to raise through put by conveying the recording paper at high speed up to a point just short of the resistance rollers and to assure that printing by the main body  1  will be performed correctly by making all of the recording paper conveyance speeds inclusive of the conveyance speeds of the optional units agree with the recording paper conveyance speed of the main body  1  at the moment the recording paper engages the resistance rollers  5 . 
     The recording paper thus supplied undergoes image transfer at the image forming section  7  and image fixation at the fixing unit  8 , after which the leading edge of the recording paper is sensed by a paper discharge sensor  29 . When the sensor  29  senses the recording paper, the engine controller  26  raises the POUTT signal  82  to logical “1”. When this is done, the master controller  101 , via the optional-unit interface  70  and after a prescribed period of time, instructs the sorter controller  103  to operate flappers so as to send the recording paper to the prescribed bin, and instructs the paper-deck controller  102  to operate the discharge-interrupting conveyance rollers  45 . 
     The POUTT signal  82  is raised to logical “1” at the timing at which the paper discharge sensor  29  senses the recording paper because the paper discharge path branches ahead of the sensor  29 . In other words, sensors are provided in the vicinity of respective paper discharge ports and a problem arises in a case where the POUTT signal  82  is raised to logical “1” at the detection timings of these sensors. For example, if the conveyance path (path B) from the fixing unit  8  to the paper discharge rollers  12  is longer than the conveyance path (path A) from the fixing unit  8  to the optional paper discharge rollers  14 , as shown in FIG. 2, a second sheet of recording paper will arrive at the paper discharge port first, via path A, while a first sheet of recording paper is passing through path B. Accordingly, though a paper discharge signal is outputted by the sensor in the vicinity of the paper discharge port at this time, this signal is that for the second sheet of recording paper. Since the order of the discharged sheets is thus reversed, it is necessary to use the detection timing of the sensor situated in the vicinity of the end of the common conveyance path. 
     The above-described paper supply operation and paper discharge operation basically are executed by having the video controller  27  verify, based upon the RDY signal  85  outputted by the engine controller  26 , that the printer engine is capable of printing, and verify, based upon the OPTRDY signal  81  outputted by the master controller  101 , that the optional units are capable of operating. However, in a case where the OPTRDY signal  81  is logical “1” and the RDY signal  85  is at logical “0”, the video controller  27  verifies the status of the printer engine via the serial communication interface  86 . If a problem such as failure to load the toner cartridge is not found, the operation for feeding paper from the paper-deck optional unit  40  is executed and the recording paper is stopped just before it is fed into the interior of the main body  1 . Thus, the recording paper can be supplied to the main body  1  promptly at the moment the RDY signal  85  attains the “1” logic level. 
     Optional-unit interface 
     FIG. 4 is a block diagram showing the detailed connections between the master controller  101  and the controller of each optional unit. 
     The master controller  101  shown in FIG. 4 is an eight-bit single-chip microcomputer. Numerals  104 ,  105  denote shift registers of eight bits, for example, for commands, and numerals  106  denote shift registers of eight bits, for example, for status. CPUs  108 ,  109  such as single-chip microcomputers control the operation of the respective optional units. Numeral  110  denotes a serial signal line Tx for commands,  111  a signal line for a signal-transmission clock CLK,  112  a signal line for a data-acceptance strobe signal STB, and  113  a serial signal line Rx for status. Information and status outputted in response to commands from the master controller  101  are stored beforehand in ROMs incorporated within the CPUs  108 ,  109 . 
     The connection and operation of each block will now be described. 
     The master controller  101  outputs commands for the optional units to the signal line Tx  110  serially connected to the shift registers  104  and  105 . The status from the optional units enters the master controller  101  via the signal line Rx  113  serially connected to the shift registers  106 ,  107 . The shift registers shift the stored data simultaneously in synchronism with the clock CLK sent from the master controller  101  via the signal line  111 . FIG. 5A is a timing chart showing the details. 
     Two bytes of data are required in order for data to be exchanged between the optional units and the master controller  101 . The data is divided into 16 periods by the clock CLK. When the transmission of two bytes of data ends, the master controller  101  issues the strobe signal STB. In response to the strobe signal STB, the CPUs  108 ,  109  accept the commands from the shift registers  104 ,  105  via parallel signal lines  115 ,  116 , respectively, and load status representing the states of the units in the shift registers  106 ,  107  via parallel signal lines  117 ,  118 , respectively. The status is transferred to the master controller  101  by the following clock pulse CLK. 
     FIG. 5B is a timing chart showing another way of applying the strobe signal STB. Specifically, when the strobe signal STB is at the “0” logic level, loading from the CPUs  108 ,  109  to the shift registers  106 ,  107  is enabled. When the strobe signal STB is logical “1”, loading from the CPUs  108 ,  109  is disabled because the data is in the process of being moved. When the strobe signal STB changes from logical “1” to logical “0”, storage from the shift registers  104 ,  105  to the CPUs  108 ,  109  is enabled. If this arrangement is adopted, one strobe signal STB can be used in an effective manner. 
     The master controller  101  includes an internal RAM  131  having a memory area for communication with the optional units. Commands and status sent and received in synchronism with the clock signal enter and exit via the memory area in real-time, thereby making high-speed communication possible. If a maximum of ten optional units can be connected, by way of example, the memory area will have a capacity of 20 bytes since two bytes are required for each optional unit to performed transmission and reception. 
     By virtue of the construction set forth above, simultaneous, high-speed transmission and reception of command and status is made possible between a plurality of optional units and the master controller  101 . 
     By repeating the transmission and reception of command and status periodically at a period of, say, 10 ms, the master controller  101  may ascertain the status of each optional unit at all times. 
     Resetting method 
     A software resetting method will now be described. 
     When resetting is carried out, the master controller  101  sends a reset command to each of the optional units. However, at the time of hardware reset, such as when power is turned on, there is a high probability that the shift-register value will be 00 h or FFh, and therefore a value having a sufficiently low probability at the time of hardware reset, such as a value AAh, is used as the reset command. Otherwise, software reset may be repeated after hardware reset. In addition, 00 h and FFh are made blank commands, namely commands which do not indicate any particular action. 
     Connection verification method 
     A method of verifying the connection of the optional units will now be described. 
     Immediately after power is introduced to the system, the order in which the optional units are connected is unclear. In order to ascertain the order, the master controller  101  executes a check of the connections immediately after the resetting operation. FIG. 6 is a flowchart illustrating the procedure for checking connections. This procedure is executed by the master controller  101  after hardware reset, such as when power is turned on. 
     As shown in FIG. 6, the resetting operation is executed at step S 1 , after which connection verification commands are issued at step S 2  in a number equivalent to the number of maximum connectable optional units. For example, if the maximum number of connectable optional units is ten but only two optional units are actually connected, as illustrated in FIG. 2, then commands for eight units flow out from the optional units and commands for the remaining two units remain in the shift registers  104 ,  105 . 
     Next, at step S 3 , the CPU of each optional unit analyzes the command and waits (several nanoseconds) for the end of preparations for output of status. A blank command is issued at step S 4  and status is sent from the shift registers  106 ,  107  is stored in the memory area for communication at step S 5 . 
     The status of an optional unit which responds to the connection verification command contains, in coded form, information indicating the characteristics of the optional unit, namely whether the unit is a paper supply device or paper discharge device, the number of paper supply cassettes and the number of sorting bins, etc. This information is read out of the communication memory area and decoded, whereby the type of optional unit and the order in which it is connected can be verified. 
     This checking of the connections is performed periodically at a prescribed period. One reason for this is to prevent erroneous verification due to noise. Another reason is to deal with cases in which the user recombines optional units while leaving the power supply turned on, or in which the power supply of an unused optional unit has been turned off. 
     Thus, in accordance with this embodiment, a wide variety of optional units can be connected without raising the cost of the main body of the image recording apparatus. Even if a plurality of optional units are combined, it is unnecessary for the user to know the order in which the units have been connected. Furthermore, since the optional master unit controls the conveyance of the recording paper, the main body of the image recording apparatus is freed from control of the conveyance of recording paper to the plurality of optional units. This makes it possible to use a variety of optional units freely in any combination without restrictions being imposed by the functions of the engine controller. 
     Further, by virtue of the fact that the master controller obtains information indicative of path length and conveyance speed relating to the feed and discharge of paper in each optional unit, each unit can be controlled at the optimum conveyance timing. Furthermore, the master controller controls the paper supply operation on the basis of the signal, which is obtained from the engine controller, indicating that feed of paper to the main body is possible, as well as the timing signal for changing over the conveyance speed of the recording paper in an optional unit to the conveyance speed of the recording paper in the printer engine, thereby making it possible to supply paper at a high speed. This makes it possible to increase thru put. 
     In addition, the master controller obtains a signal indicative of paper feed timing from a sensor situated near the terminus of the common conveyance path of the printer engine, thereby enabling the paper discharge operation of be controlled at an accurate timing. 
     Furthermore, the engine controller and master controller respectively output ready signals (RDY, OPTRDY) indicative of their respective states, and these signals are monitored by the video controller, thereby making it possible to supply and discharge recording paper accurately. 
     First Modification 
     As set forth above, paper is supplied from the paper-deck optional unit  40 . In a first modification, it is so arranged that the master controller  101  raises the OPTRDY signal  81  to logical “0” upon judging that a prescribed number of sheets of recording paper reside in the paper path within an optional unit when the video controller  27  issues the next paper feed command to the master controller  101 . Accordingly, the video controller  27  will not issue the next paper feed command until the OPTRDY signal  81  attains the “1” logic level. If this arrangement is adopted, the video controller  27  will be capable of monitoring the state of conveyance of the recording paper inside the optional unit, thereby making it possible to prevent jamming and overlapping feed of recording paper in the optional unit. 
     Second Modification 
     FIG. 7 is a block diagram illustrating another example of a manner of connecting each controller. Here the POUTT signal  82 , PFEDT signal  83  and SPCNG signal  84  issued by the engine controller  26  are temporarily accepted by the video controller  27 , which then transmits these signals to the master controller  101  as commands via the serial communication interface  80 . Adopting this arrangement makes it possible to simplify the construction of the master interface  114  connecting the video controller  27  and the master controller  101  and to reduce the cost of the main body  1  and optional master unit  32 . 
     Third Modification 
     FIG. 8 is a block diagram showing another method of connecting the master controller  101  and the controller of each optional unit. This method of connection differs from that shown in FIG. 4 in that the shift registers are used for both sending and receiving. Here shift registers  120 ,  121  are parallel-loading, parallel-output shift registers of eight bits, by way of example. Furthermore, the end of the signal line Tx is connected to the signal line Rx by a jumper line  123 . By adopting this arrangement, status which the CPUs  108 ,  109  have loaded in the registers  120 ,  121  is transmitted through the signal line Rx in synchronism with the clock CLK, and commands enter the vacant bits of the registers  120 ,  121  from the signal line Tx. Accordingly, the timing chart is the same as that shown in FIG.  5 A. 
     If this arrangement is adopted, effects similar to those of the above-described embodiment are obtained. Moreover, when optional units are combined, the jumper  123  must be attached to the connector at the terminus but the number of component parts of the optional units is reduced. This makes it possible to reduce the cost of the optional units. 
     Fourth Modification 
     FIG. 9 is a diagram showing the construction of an image recording apparatus according to a fourth modification, and FIG. 10 is a block diagram showing the manner in which each controller in FIG. 9 is connected. This arrangement differs from that of the above-described embodiment in that an interface is not provided between the video controller  27  and the master controller  101 . Rather, a master interface  38  connected by a connector  37  is provided between the engine controller  26  and the master controller  101 . 
     In a case where the video controller  27  transmits an instruction destined for the master controller  101  to the engine controller  26  via the video interface  28  and the engine controller  26  judges that the instruction received from the video interface  28  is destined for the master controller  101 , the engine controller  26  transfers this instruction to the master controller  101  as is via the serial communication interface  80 . Conversely, the engine controller  26  transfers the status received from the master controller  101  to the video controller  27  as is via the video interface  28 . As a result, the video controller  27  is capable of controlling the master controller  101  directly. 
     Further, each of the hardware signals POUTT, PEFDT and SPCNG is coupled directly to the master controller  101  from the engine controller  26 . The function of the OPTRDY signal, however, is included in the status of the serial communication interface  80 . 
     If this arrangement is adopted, effects similar to these of the above-described embodiment are obtained. Moreover, even if the connection between the video controller and the master controller is physically difficult, the video controller and the master controller can be connected via the engine controller. 
     Fifth Modification 
     FIG. 11 is a diagram showing the construction of an image recording apparatus according to a fifth modification, and FIG. 12 is a block diagram showing the manner in which each controller in FIG. 11 is connected. This modification differs from the above-described embodiment in that the master controller  101  is provided within the video controller  27 . In this case, a microcomputer  90 , which is the main control of the video controller  27 , and the master controller  101  are connected by a bus line  91 . As a result, communication between the video controller  27  and the master controller  101  can be made high in speed. 
     Further, each of the hardware signals POUTT, PEFDT and SPCNG is coupled directly to the master controller  101  from the engine controller  26 . The function of the OPTRDY signal, however, is included in the communication performed by the bus line  91 . 
     If this arrangement is adopted, effects similar to those of the above-described embodiment are obtained. Moreover, in the case of an apparatus which the user knows will have an optional unit connected thereto, it is unnecessary to independently provide an optional master unit. This makes it possible to lower the cost of the overall system. 
     Sixth Modification 
     FIG. 13 is a diagram showing the construction of an image recording apparatus according to a sixth modification, and FIG. 14 is a block diagram showing the manner in which each controller in FIG. 13 is connected. This modification differs from the above-described embodiment in that the master controller  101  is provided within the control of a specific optional unit. FIG. 13 illustrates an example in which the master controller is provided within the paper-deck controller  102 . 
     If this arrangement is adopted, effects similar to these of the above-described embodiment are obtained. Moreover, in a case where the user knows the optional unit that will be connected, it is no longer necessary to independently provide an optional master unit. This does not lead to an increase in the cost of the main body of the image recording apparatus and makes it possible to lower the cost of the overall system. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.
An image forming apparatus which includes a main body which has a printer engine which forms an image on a recording medium, a supply unit which supplies the recording medium to the main body, and a discharge unit which receives the recording medium, on which an image is recorded, discharged from the main body. The supply unit and discharge unit detachably connect to the apparatus and the units are stackable under the main body, and the supply unit and discharge unit have a relaying device for relaying the recording medium which is supplied or discharged from another unit or the main body.