How do I capture RS-232 Commands

We have a new Elevator Controller that the Manufacturer has used a 16 channel relay board (ZUXPSR1610PROXR) with the RS-232 screw connection board on (ZRS-A). They are using their standard Door Controller with special software to control the relay board to activate and deactivate buttons in the car. It works great 99% of the time but from time to time it stops activating a single relay but not always the same relay.

Reboot their door controller and it starts working again. Their Customer Support Staff have little or no training on this product (and seem unwilling to escalate the case) so I am on my own. What I want to accomplish is to “sniff” off and capture the RS-232 Traffic on my laptop with a terminal program while it is misbehaving so I can analyze it and find out if they are just not sending the command for that relay or the relay board is missing the command.

Does anybody have any good ideas on how to Sniff off the data without disturbing it?

Honestly this sounds like an induction issue. This will depend on the types of loads that are being controlled by the relays on the board. Take a look at this article:

The capacitors referenced in the article can be purchased here:

Thank You Travis.
I have a lot of experience with suppression, so when I interfaced their product to my elevator I installed the suppression ahead of time. Plus the load they are switching is actually just another 12 volt ice cube relay with a higher current capacity. So while it is an inductive load it will be a pretty small one that my suppression should have no problems sucking up.

My Spidy Sense tells me it is an issue on the serial communications side of the house.