Appendix B - Cabling, Hardware, & Electrical
User Guide
303
Straight-Through vs. Crossover Cables
The RS-232 interface was originally intended to connect a DTE (computer, printer and other
serial devices) to a DCE (modem) using a straight-through cable (all signals on one side con-
necting to the corresponding signals on the other side one-to-one). By using some “cabling
tricks,” we can use RS-232 to connect two DTEs as is the case in most modern applications.
A crossover (a.k.a. null-modem) cable is used to connect two DTEs directly, without modems
or communication lines in between. The data signals between the two sides are transmitted
and received and there are many variations on how the other control signals are wired. A
“complete” crossover cable would connect TxD with RxD, DTR with DCD/DSR, and RTS
with CTS on both sides. A “simplified” crossover cable would cross TxD and RxD and locally
short-circuit DTR with DCD/DSR and RTS with CTS.
Which cable should be used?
First, look up the proper cable for your application in the table below. Next, purchase stan-
dard off-the-shelf cables from a computer store or cable vendor. For custom cables, refer to
the cable diagrams to build your own cables or order them from Black Box or a cable vendor.
Table 25: Which cable to use
To Connect To
Use Cable
DCE DB-25 Female (standard)
•
Analog Modems
•
ISDN Terminal Adapters
Cable 1:
RJ-45 to DB-25 M straight-through (Custom). This
custom cable can be ordered from Black Box or
other cable vendors. A sample is included with the
product (“straight-through”).
