XPost: sci.electronics.equipment   
   From: presence@MUNGEpanix.com   
      
   In sci.electronics.basics Ralph Mowery  wrote:   
   > In article , presence@MUNGEpanix.com   
   > says...   
   >>   
   >> What's the control loop if the PLC dies? How do people control temperatures   
   >> manually? Is there a foot pedal to stomp on to switch the heaters on and   
   >> off?   
   >>   
   >>   
   >>   
   >   
   > The process has two PLCs running all the time. There are two or more   
   > computers hooked to the data stream for the PLCs  One PLC is the so   
   > called hot backup.  It is not controlling, but if the primary one quits,   
   > the secondary one will either switch over  automatically or it can be   
   > manually switched.   
   >   
   > One thermocouple is connected to the control loop.  If that TC goes bad,   
   > there are other TCs that give the temperature readout and the operators   
   > switch the control screen for that temperature to manual and they have   
   > to keep an eye on it and manually enter on the computer keyboard what   
   > percentage of valve opening is needed to mantain the correct   
   > temperature.   
      
   OK, so if a PLC croaks, the operators can switch to the redundant one. If a   
   probe of whatever you have between it and the current loop or whatever it   
   was fails, it's 100% manual control? How do you share the reading from one   
   problem? Not trying to pole holes in theory here, just actually curious.   
      
   One thing annoying about the probes I deal with is it seems no two are   
   alike. Never been able to swap one for another, even with short leads, even   
   with three lead compensation and get the same behavior on the same   
   controller.   
      
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