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Industrial IoT DISCUSSION
Our workshop has machines from the early 2000s. Some have a PLC with a spare RS-485 port that speaks Modbus RTU, some have only a relay panel with no communication at all. The machine builder is gone, and nobody wants to modify the PLC programs.
I need running status, cycle counts and energy use from each machine in a central dashboard. What is the usual way to retrofit this kind of equipment, and what should I watch for when connecting to the existing Modbus port?
The standard building block is an edge gateway: a small industrial computer or protocol converter with RS-485 and Ethernet ports. It acts as the Modbus master, polls the registers you configure, and forwards the values northbound over MQTT or OPC UA. The PLC program is untouched, because the PLC simply answers read requests as a slave.
You do need the register map: which address holds the cycle counter, its data type and its scaling. If the documentation is lost, the original HMI project or a read-only look at the PLC program is the usual source.
Before wiring into the spare port, find out whether something is already master on that line. Modbus RTU allows exactly one master per RS-485 segment. If an HMI is polling the PLC on the same port, a second master will collide with it and both will see timeouts and CRC errors. Options in that case are a different PLC port, a gateway designed to sit between the HMI and the PLC and share access, or adding the new functions outside the PLC.
Also restrict the gateway to the read function codes (01 to 04). Reading cannot change machine behaviour; a stray write to a holding register can.
For the relay-only machines, add sensing beside the machine rather than in its control circuit. A split-core current transformer on the main motor feed gives running, idle or off status from the current level. A three-phase energy meter with Modbus output on the machine's supply gives kWh and power, and the power curve is often enough to derive state and even cycle counts from the load pattern. A cycle signal can be taken from a spare auxiliary contact through an interposing relay or an optocoupled input, so the new electronics are galvanically separated from the old panel.
Anything inside a mains panel should be installed by a qualified electrician with the machine isolated.
Size the polling before choosing hardware. Status and counters rarely need more than one read per second, and energy values every 10 to 60 seconds are fine. At 9600 baud a request plus a 10-register reply is 8 + 25 = 33 bytes, about 38 ms on the wire with 11-bit characters, so even a slow line handles a once-per-second poll easily.
If the PLC has no counter register, count cycles in the gateway from a rising edge and store the total in non-volatile memory, otherwise a gateway restart resets the production count to zero.
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