and 2) the console monitor program. Switch 3 is the boot enable switch. When this switch is ON the bootstrap program will execute (thus loading CDOS) without first entering the monitor program. If this switch is off, RDOS begins in the console monitor mode permitting the bootstrap operation or other operations to be performed under console control. Switch 4 is the initialization inhibit switch. When this switch is ON, diskettes cannot be initialized under software control thus preventing a "runaway" program from unintentionally altering the diskette initialization. This switch must be OFF when initializing diskettes. Western Digital FD1771-1 interfaces All signals from the drives are TTL-buffered and have 150 ohm pullups. Maxi and mini signals are wired and at the pullup side of the buffers. Signals, which do not apply to the mini (i.e., READY and SEP CLOCK), are disabled and pulled high when the mini is selected. Signals to the drives from the 1771 are TTL-buffered with separate buffers for mini and maxi connectors. The STEP output is stretched by IC37 to about 16 microseconds before going to the drives. The HLD (head load) output does not go directly to the drives but rather enables the drive select lines through IC10 pin 1. Thus, the actual drive select signal to the drive is the coincidence of a latched drive selection (done at port 34H) and HLD from the 1771. Head loading time is determined by counters IC36 and 27. Timeout is controlled by the count loaded into lC3's by IC53. Signals DRQ, HLD, and INTRQ (or EOJ) are available at input port 34H (rC9). Various control signals are assigned to output port 34H and are latched by rcs 24 and 41. Board Priority Chain The 4FDC includes a ripple priority circuit which will defeat the interrupt acknowledge cycle of Priority IN/ is held low. If the 4FDC is allowed to perform the interrupt acknowledge, it will pull down its Priority Out/ line to signal others in the chain not to respond. This chain is compatible with the Cromemco TU-ART. Further Information Controller Manual in PDF Format More details of the switch setting used to configure this card as well as the theory of operation 4FDC Floppy disk computer storage