A MSG Instruction That Actually Works: Path, Type and the Error Codes

16#0204 appears twice in the same Rockwell reference manual with two different meanings, and which of the MSG instruction error codes tables you are reading decides where you spend the next hour. In the extended error codes for error 0001 it is Unconnected message timeout. In the module fault codes it is Connection Request Error: … Read more

Writing a Setpoint Back to the PLC from a Pi: The Handshake That Stops It Being a Safety Problem

The write path from dashboard to Pi to CompactLogix, with the panel selector switch drawn as a hard-wired input beside the controller and no safety tag anywhere on the path

A pycomm3 write of 95.0 to Line4_SP on a CompactLogix comes back to the Raspberry Pi with no error, and the line is now running at 95 % because nothing between the driver and the tag asked whether it should. A read-only logger becomes a control path the first time somebody puts a slider on … Read more

Safety Network Number and Signature: What Has to Match When You Replace a Safety Module

plc programming safety network number signature replace safety module 1 plc

The 1791ES-IB8XOBV4 on the palletiser cell failed on a Sunday, the spare from stores has the same catalogue number and the same three address switches set to 21, and the yellow triangle over it in the I/O tree has not moved in forty minutes. The GuardLogix 5580 project has a safety signature, the module’s Safety … Read more

One Photo-Eye, Two Signals: Light-Operate, Dark-Operate and the Setting That Inverted Your Zone Logic

One retroreflective 42EF with its reflector, beam made on an empty zone and beam broken by a carton, and below it the output and LED state of a light-operate J model and a dark-operate K model in each case

Zone 6’s eye came out of stores as a 42EF-P2KBB-F4, went onto the bracket in place of a 42EF-P2JBB-F4, and from the next 600 mm carton on, zone 6 reported occupied while it was empty and empty while a carton stood on it. One letter in the fifth position of the catalogue number, K for … Read more

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