How NOT to do serial interfacing

Warning: This story is a bit geekier than the the usual in this set, however those that understand it will get a good laugh. Those that don't, my apologies...

Back when I was working for Heath/Zenith, we had a very large customer who purchased a large quantity of our machines. Among other things, they used them as terminals for their DataGeneral systems (a big mainframe system). Each desk in their office had a serial cable running to it.

While they were very happy with our desktops, they did not like our laptops, claiming they were incompatible with their terminal emulation software. This went on for quite some time, and we found it very strange that EVER SINGLE ONE of our highly regarded laptops were incompatible with this application, yet every single desktop we sold worked fine.

One day, we got a call, and they told use that the problem was bigger than just our laptops. They had a Toshiba on loan for evaluation, and it had the same problems as our machines did. Finally, they took our salesperson up on his offer to send me out to look into what was going on.

I showed up with my home-built breakout box, which was NOT a thing of beauty, but VERY functional and one of our laptops. One of my first technical tasks I did when I started working at the store was interfacing a serial printer to one of our computers -- I really understood this very well, and I was quite confident I would have some idea what was going on very quickly, but pulling out this ugly mess of wires and LEDs didn't inspire great confidence, but it did inspire a bit of curiosity.

So, I explained how this set of bi-color LEDs would monitor lines I hooked them to, I explained how this wire was connected to the ground line, etc. To start with, I was going to just look at the Send and Receive data pins, "and of course, use the signal ground".

"You don't need that", one of the IT people responded.
"Oh, yes we do!", I corrected.
"Oh, no, we only use the 2 wire serial interface"
"There is no such thing"
"Sure there is, that's what we use everywhere here!"
"What two pins?"
"2 and 3", they responded, the send and receive data lines.
"You still need the ground!" I responded.
"Oh, no, we just use the common building ground as the signal return line"
In other words...they were using the ground pin of the computer's power cord as the signal ground. Bad idea.
"That can't work all the time", I responded
"Sure it does! All of our machines work fine!"
You see, laptops are DC isolated from the building ground -- many laptops don't even have a third pin on their power cords. Thus, there was no signal ground for the laptops. As this is a near universal design, almost all laptop manufacturers would have the same issue in this building.

After a couple minutes of demonstrating that adding a ground wire to the laptop resulted in perfect operation, suddenly something else occurred to me: After every large electrical storm, this customer would bring in a VAN LOAD of broken computers -- every computer had the same problem -- blown serial ports. This was particularly strange, as our machines were very well built and rugged one else brought in computers like this, just this one customer. We had both written it off to "strange electrical problems in the building". Suddenly, the cause became clear.

When I had finished demonstrating the problem, and explaining why they needed a minimum of a three-pin connection, one of the two IT guys said, "I *knew* that was wrong. I just couldn't remember why!"

Copyright 2002, Nick Holland
Return to War Stories
Return to Nick Holland's Home Page