wer printer can print, it will drop the DTR lead, causing the PC to temporarily halt its transmission of the print file. As soon as the printer is ready to receive again, it sets the DTR lead high, and printing resumes. As long as both the transmitter and the receiver abide by this standard set of rules, data communications will work properly. This process of swapping the data on the various leads of a cable, incidentally, is done by the modem-or by a null modem cable Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. First Things First First Things First Figure 1-2 Send Data (pin 2) Ground assignments on a cable Receive Carrier connector Data Set Ready Request to Send Data Terminal Clear to Send Ready Figure 1-3 Logical wiring scheme that makes the communicating devices think they are talking to a modem. The null modem cable is wired so that the send data lead on one end is connected to the receive data lead on the other end, and vice-versa. Similarly, a number of control leads-such as the carrier detect lead, the DTR lead, and the data set ready (DSR) lead-are wired together so that they give false indications to each other to indicate that they are ready to proceed with a transmission, when in fact no response from the far end modem has been received. Standards: Where Do They Come From? Physicists, electrical engineers, and computer scientists generally design data communications protocols. For example, the Transmission Control Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. First Things First Chapter 1 Protocol (TCP) and the Internet Protocol (IP), discussed in detail later in the book, were written during the heady days of the Internet, back in the 1960s, by such earl