Black Box ACS120 User's Guide download pdf (Page 303)

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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”).

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