A PLC, or Programmable Logic Controller, is an industrial computer. Wikipedia does an excellent job of explaining what a general PLC is
Programmable logic controller - Wikipedia, the free encyclopedia
The Allen Bradley 1785-L60B or PLC 5/60, has several cards installed for my purposes, each with a different type of electrical interface. It is set up like this:
CPUI PLC 5/60
Slot 0 - Power supply
Slot 1 - spare
Slot 2 - 1771-IBD
Slot 3 - 1771-IBD
Slot 4 - 1771-OBD
Slot 5 - 1771-OBD
Slot 6 - 1771-IK
Slot 7 - 1771-IK
Slot 8 - 1771-IFC
Slot 9 - 1771-IFC
Slot 10 - spare
Slot 11 - spare
Slot 12 - 1771-OFE
Slot 13 - 1771-OFE
Slot 14 - KE communications card
Slot 15 - KE communications card
Here's some information on the various cards, the voltages and currents
DC Input - 10 - 30 VDC. 1 ms response time rising or falling. 4.5 mA at 10 VDC, 12.5 mA at 24 VDC, 30 mA at 30 VDC to properly turn on the input. Sinks a minimum of 2.0 mA when off. 0 -> 5 VDC is off, 5 - 10 VDC is last state, 10 - 30 VDC is on. The temperature spec is 0C - 60C, but I know of several installations where it runs well down to -25C.
DC Output - 10 - 60 VDC. 0.1 ms delay rise time, 0.2 ms delay falling. 2.5 mA to fully turn on. Leakage less than 0.5 mA when off. 25A max output pulse for 10 ms. 2A continuous. Rated at 1.5 VDC voltage drop across output at rated current.
Encoder Input - 12 - 24 VDC. Single channel quadrature encoder counts to 50Khz. Single 12-bit count sent to the PLC per channel, with underflow and overflow.
Analog Input - 0 - 5V, 0 - 10V, -10V - 10V, 4 - 20 mA analog input module, 12 bit resolution, 4 channel, common negative signal.
Analog Output - 4 - 20 mA analog output module, 12 bit resolution, 8 channel, common negative signal.