Mysterious reboot every wednesday
This is a dedicated machine built around a Nova computer ( i'm talking mid 80's) used for IC layout.
This machine has a couple of diablo disk drives and is hooked up to specialistic terminals (Calma)
At a certain point in time we are baffled by the following : every thursday morning the machine is found to have gone through a full reset. Service is called. They are baffled too. These machines do not a have a watchdog , nor a realtime clock and there is no known software that can cause a timed reboot. One wednesday evening the machine is booted from a clean diskpack and left in a steady state with nothing running on it . And then the wait begins. Sure enough at a certain point : reboot.
Every possible avenue is tried : back-up UPS investigated, computer investigated , peripherals investigated.
Ultimately the diskpack is sent off to the manucfacturer where they insert it in a brand new machine, wait for wednesday/thursday turnover and nothing happens over there.
suspecting a serious hardware problem in the machine they ship an entire new machine. Machine gets installed, wired in booted and then the wait begins. Come wednesday evening ... nothing happens. All is well. Next wednesday .. all is well. Looks like the machine was faulty. Then the week after : it starts again.... So : someone camps out in the computer room ( the fishbowl we called it. a room surrounded in glass panels , with an adjacent room where the line printer and the large format flatbed plotter ( we are talking A0 sheets here ) is. To get into the computer room you need to go through the printer room and only select people have access to the computer room.
come wednesday evening late at night : the cleaning crew comes in. One lady opens the printer room door, plugs in the vacuum cleaner in the socket over there and proceeds to vacuum. The Nova reboots.
How can this be ? the fishbowl has its own supply , on a double redundant UPS, isolated from the rest of the building. That socket is not in use for anything else in the computer room. We even trace the wiring and the fuse feeding that socket is not shared with anything else. It is just a utility socket for just such purposes : provide 'dirty power' if any work is to be done.
So we lift the floortiles ( raised computer room floor ). Lo and behold: there is the cable connecting the plotter to the computer. 1 inch underneath the power socket.
What happened : switching on the vacuum cleaner caused a spike on the comms cable between plotter and computer causing the computer to freeze and reboot. We had changed cleaning crews a while ago. They were given explicit instructions not to plug anything into the orange sockets around the computer room ( clean power on ups ) , and this was a white socket. It turned out the trouble started after the new cleaning company took over. Then why did the new machine work fine for 2 weeks ? the vacuum lady was sick and her replacement simply used an extention cord to reach that area.