RS232 serial ports are normally found in two types: DTE (data terminal equipment) or DCE (data communications equipment) interfaces.
These describe the signal direction of the interface.
When building a cable you must be aware of the signal direction of both RS232 interfaces so that correct signals can be wired.
A DTE serial interface would be cabled straight through to a DCE serial interface, while a DTE serial interface would be cabled in a cross over configuration to another DTE interface.
A breakout box can be used to monitor the signals between two serial devices. By connecting the serial device to a breakout box you can figure out what type of interface it is by viewing the hardware signals:
- A DTE interface will raise (output) DTR and/or RTS
- A DCE interface will raise (output) DSR and/or CTS.
Example: Iolan DS RS232 DB25 DTE serial interface to PC RS232 DB25 DTE serial interface will use a cross over cable.
Some RS232 serial interfaces may not be standardized especially if they are not DB25 or DB9, such as the Jetstream or CS9000's RJ45 RS232 interface.
In this case you should mate the signals based on the signal directions. Input to Output and match the signal pairs.
TXD / RXD
DTR / DSR
RTS / CTS
Example: CS9000 RJ45 serial interface to DB25 RS232 DTE serial interface:
2 DSR output ---> 6 DSR input
3 DTR input <--- 20 DTR output
4 GND ---- 7 GND
5 TXD output ---> 3 RXD input
6 RXD input <--- 2 TXD output
7 CTS output ---> 5 CTS input
8 RTS input <--- 4 RTS output
Note: some serial devices may require to see DCD (data carrier detect) high before responding on the serial port. You should then wire the device's DCD pin to another pin that is outputing a high signal (such as DTR).