Splitting serial port output

Question

I have a nurse call system that has output that connects to a paging transmitter. Works good right now. We put in a serial "Y" splitter so that we could receive the same data, but to another computer for capturing. The original side that goes to the paging transmitter works flawlessly UNTIL I plug the other side up to a computer/device. Then the original side starts spitting out gibberish characters. Here's the weird thing though, when I do plug up the other (new) side to a PC, it outputs the correct data.

The original side uses a serial cable to connect the nurse call system to the paging transmitter, so I just assumed I should use the same on the other (new) side. Baud rate matches on all devices, 9600 8N1.

Looking for any kind of advice or maybe something to look for.

Answer 1

Maybe the "new" PC serial connection is affecting the logic levels of the signal and corrupting the paging device's read.

Maybe one of these Serial Port Splitters might be better designed to properly tap the serial traffic.

Is it an option to install a Serial Port Monitor on the host computer and use that to log serial traffic?

Answer 2

RS232 serial data lines do not like being split. They do not support "multi-drop" connections similar to what you are trying to attempt.

They do not work because RS232 signal levels expect only a single receiver on each side and that receiver to have a specific impedance. When you connect multiple receivers then the signal level is attenuated and goes out of specification. As the signal is no longer guaranteed to reach the required level it is only sometimes detected as the logic levels being transmitted and you get garbage.

If you want a multi-drop communication method then you will need to convert the signal to something else. RS485 supports multi-drop and you can buy RS232 to RS485 or USB to RS485 converters which use a signalling protocol and termination method that is much more amenable to your use case.