PLCs share a modular construction with stacked PCBs (Figure 1):
- Base: Power / I/O PCB
- Middle: Logic board with microcontroller and communications interface (e.g., Ethernet)
- Top: The PLC may include a display and pushbuttons
Figure 1: Image of a PLC with cover removed showing the stacked PCBs.
Modular Construction
Most OEMs produce families of PLCs. They differ in output type (relay vs solid state) and power supply (24 VDC vs 120 VAC). These options are typically incorporated into the base power / I/O board. This allows the OEM to swap the base board as required. As a result, the logic PCB and the case require minimal (or no) modifications. For example, the power board for the Siemens LOGO! shown in Figure 2 has 24 VDC power supply input, 24 VDC inputs, and four output relays.
Figure 2: Image of the PLC circuit boards power / output (left) logic (right) and HMI (middle).
Isolation of the Sensitive Logic
The power / I/O is designed to protect the logic board from hazards of the industrial environment. It’s like a cocoon surrounding the microcontroller. For example, the Texas Instruments Tiva C microcontroller as shown in Figure 3 has a 3.3 VDC interface. Optical isolation is typically used to level shift between the voltage domains and provide galvanic isolation with a measure of protection against static electricity.
Repair Implications
Modular construction is an advantage for those who repair the PLC. Worn relays and damaged I/O semiconductors can be replaced. Damage such as inadvertently applying 120 VAC to a 24 VDC bus may be contained as opposed to spreading destruction into the logic board.
Don’t read too much into this statement. Crossing 120 VAC into the 24 VDC supply terminals will result is catastrophic damage that could ripple through the entire PLC.
Figure 3: Image of the Texas Instruments Tiva C microcontroller on the logic PCB.
Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey. Author bio.