RS-232 Information and Troubleshooting Tips
The following is information related to RS-232 communication and connections, mainly involving the connection of RS-232 devices to the serial ports of IBM PC compatible computers.
IBM PC Compatible Serial Port Pinouts
signal flow pin pin on PC (DB9M) (DB25M) DCD in <--- 1 8 receive <--- 2 3 transmit ---> 3 2 DTR out ---> 4 20 asserted when port is opened ground 5 7 DSR in <--- 6 6 RTS out ---> 7 4 asserted if hardware handshake CTS in <--- 8 5 must be held asserted to send if hardware handshake RI in <--- 9 22
Looping back Control SignalsMany RS-232 devices that connect to a PC will work with only transmit, receive, and ground connections. If they do not, this may mean that they require that some or all of the control signals be connected and working. This will probably require using a cable with all of the wires present. Another way to simulate these signals to the PC is to loop back the PC's own signals as shown below.
Monitoring RS-232 CommunicationThere are times when there is just no substitute for seeing exactly what two devices are sending to each other when they are failing to communicate properly or are not working as they should be. Shown below is a device that will allow you to "eavesdrop" on an RS-232 communication with a dumb terminal or IBM PC compatible computer running a communication program.
The switch allows the monitor PC to select which side of the "conversation" it is receiving and displaying.
The device shown uses 9 pin "D" connectors and is made for one monitor PC, but the following modifications can be made for your particular situation: