Today was a service window, and I was working in an old part of the network. I stumbled onto a broken DE9-toRJ45 adapter, dangling by it's wires. I swapped it out for a new one, from the proper vendor for the terminal servers in use, but there was no response via conserver.
"It was working fine last week." I'm told.
Looking further, I found a poor RJ45 termination, but the signals on on my signal tracer looked odd as well. This led me to the other end of the cable, where I found that I was looking at a USOC-Rolled cable (old CDDI-type wiring). This would have been a 'null-modem' if they had been using Cisco console servers, but they were not. Instead, they had only a couple leads correctly connected, and signal ground was not one of them. After a bit of frustration reaching more consoles, and reterminating those cables to be straight-through again, the network was back under our control. But why had they failed?
"But they've always worked like that." I'm told.
The wiring of the Cisco adapters, the null-modem (USOC-Rolled) cables, and the non-Cisco console servers wiring scheme had meant that signal reference was really be derived from the equipment chassis, across the power systems ground, between the console servers and the controlled devices. Today was they day that newer, fatter power circuits were installed, and moving the gear to different Power Distribution Units were too much.
The signal reference ground pin is extremely important, to prevent data errors, possible equipment damage, and possible injury to staffl handling the connections! (I'll share my 'bad ground, smoking gun' story another day, when I have more time.)
Using the correct adapters, and correct wiring, will lead to predictable results. Mixing and matching parts you have laying around with specialty cables will eventually cause you more headaches, and possibly more money than you will save. And the chances are good that you'll need to 'do it the right way' sooner or later. If your shop is as busy as the places I've been, you may as well do it right in the first place, because you may not have time to do it right later without sacrificing weekends or vacation time.