With transmitter at 20 mA output, measure voltage at the transmitter terminals. Must be ≥12 V DC for most three-wire devices (check datasheet). For loop-powered transmitters, the drop can be as low as 7.5 V.
The standard defines accuracy classes for analog transmission:
A CIE 542-compliant loop must reject common-mode noise up to 100 V RMS (50/60 Hz) and series-mode noise of 10 mV peak-to-peak without exceeding ±0.1% error.
To ensure a process loop meets CIE 542 specifications, regular testing is mandatory. Here’s the standard test protocol:
The IEC has officially replaced many analog standards with IEC 61158 (fieldbus) and IEC 62769 (FDI integration). However, no international body has deprecated 4-20 mA. In fact, new products are still designed to CIE 542 specifications.
The latest trend is Ethernet-APL, which carries 10 Mbit/s Ethernet over two wires up to 1000 meters. Nevertheless, APL gateways include multiple 4-20 mA analog outputs specifically to bridge old and new worlds. The death of CIE 542 has been predicted for 40 years—and it remains as ubiquitous as the 24 V DC power supply. cie 542
To appreciate the importance of CIE 542, we must travel back to the post-World War II industrial boom. Before standardized analog signals, every manufacturer used proprietary signal ranges (e.g., 1-5 V, 0-10 V, 10-50 mA). This created chaos: a Rosemount transmitter could not drive a Honeywell recorder without expensive signal conditioners.
In the 1950s and 1960s, the International Electrotechnical Commission began work on what would become IEC 60381 and its precursor documents. In certain CENELEC (European Committee for Electrotechnical Standardization) and national archives, these specifications were cross-referenced with CIE document numbers. CIE 542 specifically emerged as a French and Italian designation for "Transmission de signaux analogiques pour systèmes de commande de processus" — the living document that enshrined the 4-20 mA current loop as the universal analog communication standard.
While the official number has been superseded, "CIE 542" persists in legacy system manuals, technical exam questions, and older maintenance logs.
When a CIE 542-compliant loop fails, follow this workflow:
Step 1 – Check supply voltage at device terminals. With transmitter at 20 mA output, measure voltage
Step 2 – Measure loop current with a multimeter (ma mode).
Step 3 – Perform a loop calibration test. Isolate the transmitter and apply a known input using a process calibrator (e.g., Fluke 789). Compare actual mA to expected from CIE 542 linear relationship:
Output (mA) = 4 mA + (Input % × 0.16 mA)
Step 4 – Check ground loops. CIE 542 assumes a single point ground. Multiple grounds cause mysterious shifting. Measure AC voltage between signal negative and earth ground – should be <100 mV AC.
Step 5 – Test receiver input. Disconnect transmitter and substitute a variable resistor or mA source. Sweep 4-20 mA and monitor the display/PLC value. If mismatch persists, the receiver is faulty. Step 2 – Measure loop current with a multimeter (ma mode)
In the world of industrial instrumentation, process control, and automation, standards are the silent guardians of safety, reliability, and interoperability. Among the myriad of technical documents published by the International Electrotechnical Commission (IEC) and other bodies, the term CIE 542 frequently surfaces—often surrounded by confusion.
First and foremost, a critical clarification: CIE 542 is not an active, standalone standard published by the International Commission on Illumination (CIE), which typically handles lighting and colorimetry. Instead, "CIE 542" is most commonly an industry shorthand, a typographical variant, or a legacy reference to IEC 60542 (formerly known as CIE 542 in certain European documentation systems, particularly in French or German contexts).
For the purpose of this article, CIE 542 refers to the harmonized specification for "Direct current and low-frequency analog signals for process control systems" — specifically the 4-20 mA current loop standard, as codified in historical CIE/IEC documentation.
Understanding CIE 542 is essential for process engineers, control system integrators, instrumentation technicians, and anyone working with pressure transmitters, temperature sensors, actuators, and PLCs (Programmable Logic Controllers).