Sir, is the 250 Ohm resistor typically inbuilt within the barriers located in the marshalling cabinet, or is it provided inside the PLC/DCS I/O modules? In cases where barriers are not used, where should the resistor be considered or installed?
The 250 Ohm resistor is primarily used in analog input loops, especially when converting a 4–20 mA current signal into a 1–5 V voltage signal for the control system (PLC/DCS) — typically the Analog Input (AI) module of a PLC or DCS.
Where is the 250 Ohm Resistor Located?
Inside Barriers (Marshalling Cabinet) – YES, Often Inbuilt
In intrinsically safe (IS) loops, signal isolators or barriers installed in the marshalling cabinet often include a built-in 250 Ohm resistor as part of their internal circuit. This ensures the signal is safely converted and isolated for use in hazardous areas.
Inside I/O Modules (System Cabinet) – YES, in AI Cards
In many PLC/DCS system analog input modules, the 250 Ohm resistor is internally integrated and can often be enabled or disabled through hardware jumpers or software configuration.
Note: This depends on the make and model of the system. Not all AI cards include an internal resistor, depends on the model.
Conclusion:
So, the 250 Ohm resistor can be located in either/both:
-
the barrier module (in the marshalling cabinet), or
-
the AI module (in the system cabinet),
depending on system design and safety requirements.