Bit that doesn't go anywhere

Wilso

Member
W
Join Date
Nov 2012
Location
Newcastle
Posts
144
This problem has stumped the engineers available to me. The bit in question - M403.4 is not shared data, only refers to itself in the search, yet is ultimately responsible for the activation on the output Q91.4.
How could it be activated?

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It's definitely not global data. There's nothing even close to that range of addresses.

How could it come from a HMI?
 
Should point out that this bit is just one example. There's 33 steps in all in this state machine(?), and we've got no idea how each state is generated.
 
Is there anything other than the HMI that could be writing bits? Like a servo controller IO card? Or some other IO card in the rack that the PLC program itself doesn't refer to, just reads the bits & words written by it?
 

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