Unwanted signals going through RS-232 port when Analog I/O channels are being used. Why and how to solve the problem?

[Mr.T]

My application involves operating a pH meter via RS-232 protocol. Other hardware are operated using an E-series DAQ board whereby waveforms are software-generated and sent to the relevant hardware using an Analog Output channel. Data acquisition is performed utilising Analog Input channels of the DAQ card that have been configured in a Referenced Single-Ended mode. When the Analog I/O channels are active erroneous readings are observed on the pH meter if the RS-232 cable is connected between the pH meter and the control PC. No problem is observed if the RS-232 cable is disconnected. What could be the problem and any suggestions on how to solve this.

[Chris_Rake]

A couple of questions:

1) What brand RS-232 interface are you using? (built-in, NI, etc.)
2) What bus type are you using for your RS-232? (built-in, PCI, USB, etc.)
3) Is your RS-232 cable shield connected?
4) Is your RS-232 digital ground signal connected?

[Mr.T]

Comments to your questions:

Built-in = Originally supplied on the PC's motherboard

1)I use built-in RS-232 interface
2)I use built-in Bus type
3)The RS-232 cable is shield connected
4) I'm not sure on how to check if the RS-232's digital ground signal is connected. I simply connected the RS-232 cable from the pH meter to the serial port. The serial communication works fine, with no disturbance to the pH meter if i don't initiate data acquisition.

[Chris_Rake]

An easy check ground is to use a multimeter (or ohm meter). You should see near zero (<1 ohm) resistance between pin 5 and ground (you can use the chassis though its better to use a known-ground on the motherboard).

Its rare for the MB to be incorrect (but could happen). Other things to try include moving the cable, moving the card further away from the COM port, and trying a different RS-232 cable (perhaps the cable you're using is unshielded).