A refinery upgraded its aging Distributed Control System to a modern PLC based automation platform while keeping the existing smart transmitters and HART multiplexer infrastructure for diagnostics and predictive maintenance. After commissioning, operators observed that all pressure, temperature, and flow values were displayed correctly in the PLC and trending normally in the SCADA system. However, the asset management software connected through the HART multiplexer suddenly lost communication with several field instruments.
Maintenance engineers initially suspected transmitter hardware failure because the HART multiplexer could not retrieve device diagnostics, secondary variables, or status alerts. Surprisingly, when technicians connected a handheld HART communicator directly across the transmitter terminals in the field, communication worked perfectly and all transmitter parameters were accessible.
Further investigation revealed that the newly installed PLC analog input cards had very low internal impedance compared to the previous DCS cards. The old DCS input cards naturally provided enough loop resistance for HART communication, while the new PLC cards only supported the analog 4 to 20 mA current signal. Because of this, the HART Frequency Shift Keying signal became too weak for the multiplexer to detect reliably.
The engineering team installed an external 250 Ohm resistor in series with the analog loop. Immediately after adding the resistor, the HART multiplexer restored stable communication with all transmitters, and the AMS software began receiving diagnostics and device health information again.