I recently had to interface with some old lab equipment. It had a db25 pin serial connector. The serial card for the instrument was based on the Intel 8088. When I hooked up all the serial lines on the connector, there were still a couple of pins left over on the connector that had traces connected to them.

The first was pin 14 of the db connector. It was attached to the output of a serial driver to PCR5 (port B strobe) of the Intel 8155H Mos Ram.

The second was pin 18 of the connector. It was attached to the input of a tied low not gate to the NMI pin of the 8088.

Basically I am wondering what the functionality/use of these pins might be in controlling my instrument and whether it would make sense to connect them to my serial cable somewhere. I did read that the NMI was sometimes used for power failure protection, but I suppose the interrupt could trigger any subroutine. As for the strobe, I have no idea. Pins 1 (carrier detect) and 9 (ring indicator) are still available on my serial cable.

 

Draw the block diagram of connectors and your equipment and labeling the wiring.

 

Here is a picture of the situation. I am wondering why the manufacturer of the instrument included these pins. Perhaps they were just for factory testing, but I don't know.

 

RS232 Data Interface - a Tutorial on Data Interface and cables.
P.14 - S.TXD - Secondary Transmit Data
P.18 - LL - Local Loop Control

P.18(DB25) → ↑▔▏ → 74LS02 → ▕▔↓ → NMI(8088)

I think that two pins could be used to do some kind of the loop self test for the equipment, the signal is send from 8155 and receive from P.18 to NMI(8088), you can using a wire to shorted the pin 14 and 18 to see how the equipment will be happening.

 

I suppose that could be correct. If I were to get it wrong though and the NMI pin were to be toggled, it would pretty much disable the device in that it would be running a routine that it shouldn't be. I will give it a shot though.

 

Usually the NMI was designed for the syetem to run a special routine immediately and has most priority interrupt.