Designing Embedded Hardware by John Catsoulis

Author:John Catsoulis
Language: eng
Format: mobi, epub, pdf
Tags: COMPUTERS / Hardware / General
ISBN: 9780596007553
Publisher: O'Reilly Media
Published: 2003-01-02T00:00:00+00:00

Figure 9-5. DTE device connected to a computer

Figure 9-6. DCE device connected to a computer

Manufacturers, when faced with this problem, arbitrarily chose one or the other. The IBM PC has a DTE-type connector, whereas the makers of Unix workstations (such as Sun Microsystems) often choose to make their machines with DCE connectors, since they are more likely to be connected to terminals. To connect a PC to a modem, you need a DTE-DCE cable. To connect a PC to a terminal, you need a DTE-DTE cable. To connect a Sun workstation to a terminal, you need a DCE-DTE cable. To connect a Sun to a modem you need a DCE-DCE cable. To connect a Sun to another Sun, you need a DCE-DCE null modem cable (where Rx and Tx cross over), and to connect a Sun to a PC, you need a DCE-DTE null modem cable. If, however, you need to connect two PCs together, you need a DTE-DTE null modem cable. So, for just two types of device (DTE and DCE), you need six types of cable to cope with the permutations! Variety, as they say, is the spice of life, but it's the bane of RS-232C!

Table 9-1 shows the "standard" connections for RS-232C, for both 25-pin and 9-pin connectors. The signal names are DTE-relative. For example, Tx refers to data being transmitted from the DTE but received by a DCE.

Table 9-1. RS-232C signals

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