dos loop back test

[gerty]

We're working on an old irragation system here at the school and it has a dos based control.
It was manufactured by someone that passed away 2 years ago. It uses an Opto-22 relay board with an opto-22 485 card in the computer: http://www.opto22.com/site/pr_details.a ... 1006,,,3,6&
The system quit 2 weeks ago, it had preset time to water the nursery stock ,or not to turn the water on if it was raining.
The question is, the 485 card was replaced ,because we saw no transmit or recieve traffic on the indicating leds , and it still doesn't work. I have used loopback plugs in the past but never on a dos system. Is there a dos command for this?
When we first looked at this I didn't pay a whole lot of attention to the pc and therfore don't even know if it has a cd drive in it, nor do i know what version of dos it's running.
We really need to determine what the bad component is and see if it can be repaired/replaced.
The only real sympthom it has is no communications. The OPto relay board has toggle switches for manual overide and they work.

[Bob Scott]

Is there a serial cable involved? Check continuity for a broken connection in it.

[gerty]

The original cable is run underground from another building. When we were called over they had already taken the computer over to the greenhouse and plugged it directly into the opto22 relay board,bypassing all the original cables. Wouldn't the transmit led on the 485 card in the computer come on, even without a cable connected ?

[reloadron]

This link may be worth a look. They show illustrations for making the connector for loopback. Since you have replaced the 485 card this will at least check the serial communications of the system using a loopback. Minor hoop to jump through to get to the page. I started HERE and scrolled down to the FAQ to get the above link.

I would think you would see Tx activity on the card LED if the card was transmitting. Could be a problem with the system executing the code.

Ron