RMA
Member
Reading the interesting backgrounds of some of you in the "So how'd you find yourself programming PLCs?" thread, it occurred to me that there must be quite a few good stories out there waiting to be told. Just to start it off, here are a couple of things that happened to me along the way.
The first one really was a case of "Gee, that was close!". In the early seventies I was involved in doing maintenance on a Kent K70 system in Gorzow, in Poland. The situation was always the same, the customer would phone in and complain that something trivial wasn't working, from experience we new that there would be more to it than that and after a bit of pressing gradually we would get a better idea of what all was faulty (but never 100%), so we always went well armed with whatever spares we could find and loaded up with ICs etc., not to mention "emergency rations" in the form of tins of baked beans, bars of chocolate and so on, because getting fed could be a bit erratic!
The really interesting bit though, was that every time you crossed over the border at Franfurt an der Oder, you knew you weren't coming back out until everything was working - no Exit Visa, (a bit like Iraq 10 years later, I discovered)! On this occasion one of the faults turned out to be the Timing Card in the DEC PDP8 CPU - and that was when I started to sweat a little. Although we regularly repaired all our equipment down to chip level, for some reason we had never managed it with the Timing Card in the factory and always finished up sending the board back to DEC. That of course was on the basis of hitting a point where the time spent was going to cost more than the exchange card - this time I was going to have all the time I needed!
The Timing Card was not particularly exotic, just lots of single and dual monostables daisy-chained to give several strings of phase shifted clock pulses. So I settled down with the 100 MHz Tek (analogue, thank God!) having pulled down the blinds and turned off the lights to go chasing one-off, sub µ-second pulses. After having carefully unsoldered and replaced several ICs (and then re-unsoldered and replaced the original if that was not the problem - I didn't have that many spares!) I eventually traced the fault to a particular 74123 dual monostable. However, when I swapped my brand new spare in, it still didn't work. Looking a bit closer, I realised, that the DEC designer, for reasons best known to himself, had arranged the circuit so that the Trigger conditions on the input were (at that particular moment in time) permanently enabled and the pulse was actually fired by releasing the Reset line, which was otherwise permanently held on. Anyway, the problem was easily solved by swapping one from somewhere else on the board where it was triggered conventionally and replacing that one with my new spare.
That bought me my freedom and back in the office I went chasing the cause of the problem - it didn't take long to find it. At that time, TI was the de facto standard for the 74-series ICs and these were used on the DEC board. Our design office had also specced TI chips, but somwhere along the line, the bean counters had decided to save a few pennies and had ordered National Semiconductor "compatibles". So I phoned up Nat Sem to complain to be met with the reply "Who on earth would design a circuit to work like that?", to which the only answer was "DEC"!
We went back to TI after that, but some years later when many so-called VT52 and VT100 compatibles had problems, because DEC didn't stick to their published specs, I often wondered if TI might have been playing the same game.
I think that's enough for one posting, I'll post my other one, which thankfully fell more into the "Gee, that's neat" category, later, if the thread takes off.
The first one really was a case of "Gee, that was close!". In the early seventies I was involved in doing maintenance on a Kent K70 system in Gorzow, in Poland. The situation was always the same, the customer would phone in and complain that something trivial wasn't working, from experience we new that there would be more to it than that and after a bit of pressing gradually we would get a better idea of what all was faulty (but never 100%), so we always went well armed with whatever spares we could find and loaded up with ICs etc., not to mention "emergency rations" in the form of tins of baked beans, bars of chocolate and so on, because getting fed could be a bit erratic!
The really interesting bit though, was that every time you crossed over the border at Franfurt an der Oder, you knew you weren't coming back out until everything was working - no Exit Visa, (a bit like Iraq 10 years later, I discovered)! On this occasion one of the faults turned out to be the Timing Card in the DEC PDP8 CPU - and that was when I started to sweat a little. Although we regularly repaired all our equipment down to chip level, for some reason we had never managed it with the Timing Card in the factory and always finished up sending the board back to DEC. That of course was on the basis of hitting a point where the time spent was going to cost more than the exchange card - this time I was going to have all the time I needed!
The Timing Card was not particularly exotic, just lots of single and dual monostables daisy-chained to give several strings of phase shifted clock pulses. So I settled down with the 100 MHz Tek (analogue, thank God!) having pulled down the blinds and turned off the lights to go chasing one-off, sub µ-second pulses. After having carefully unsoldered and replaced several ICs (and then re-unsoldered and replaced the original if that was not the problem - I didn't have that many spares!) I eventually traced the fault to a particular 74123 dual monostable. However, when I swapped my brand new spare in, it still didn't work. Looking a bit closer, I realised, that the DEC designer, for reasons best known to himself, had arranged the circuit so that the Trigger conditions on the input were (at that particular moment in time) permanently enabled and the pulse was actually fired by releasing the Reset line, which was otherwise permanently held on. Anyway, the problem was easily solved by swapping one from somewhere else on the board where it was triggered conventionally and replacing that one with my new spare.
That bought me my freedom and back in the office I went chasing the cause of the problem - it didn't take long to find it. At that time, TI was the de facto standard for the 74-series ICs and these were used on the DEC board. Our design office had also specced TI chips, but somwhere along the line, the bean counters had decided to save a few pennies and had ordered National Semiconductor "compatibles". So I phoned up Nat Sem to complain to be met with the reply "Who on earth would design a circuit to work like that?", to which the only answer was "DEC"!
We went back to TI after that, but some years later when many so-called VT52 and VT100 compatibles had problems, because DEC didn't stick to their published specs, I often wondered if TI might have been playing the same game.
I think that's enough for one posting, I'll post my other one, which thankfully fell more into the "Gee, that's neat" category, later, if the thread takes off.
