Can't alter program A series.

6 posts in this topic

A strange one yesterday so I am asking if anyone has come across such a thing.


A customer bought a second hand machine and they wanted me to do some minor alterations to the program.

It has an A series plc and a beijers e100 hmi (so you can tell it's old)

I uploaded the plc program with no trouble.


The machine has 12 identical stations and all the I/O go back to 12 seperate PC boards. These boards then plug into the plc with 12 plugin leads.

All the I/O are digital - no analogue. It looks like it is just an interface board but there are some pretty big IC's on these boards.?

I monitored the program and couldn't find a lot of the inputs in the program.

For instance: Station 1, start button =X1, press the start button X1 input lights up on the plc. X1 does not exist in the program but the station starts

The same with them all.

When the machine is totally stopped, lots of inputs and outputs are flashing on and off at a rapid rate.

If I try alter anything it just won't write to the plc

The machine works perfectly, it's not lose wires or anything.







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4 minutes ago, Ron_S said:

machine has 12 identical stations and all

The stations you mention are remote i/o right. So they don't have to be X or Y.
Which network is it CC-Link ?

when you alter something you can't write it into the PLC.... what happens exactly.
You don't have communication ? do you get an error ? 


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To describe it better, the 12 stations are all one machine on the same common frame.

They all do the same job times 12. (Wind yarn into cones to an exact length.)

The I/O come into the panel from each station in a multicore and connect to these printed circuit boards. (Inputs to 1set of terminals outputs to another)

These pc boards then have 2 plugs connected to the plc I/O (the standard Mitsubishi plugs that connect to the input and output units)

There is nothing complicated about this machine. It has buttons, limit switches, valves  and photo cells. And a motor on each station controlled by an inverter that only has 2 inputs (slow and fast),

I can't quite remember if there was an error when I tried writing a line, it went grey and when I pressed write, it just stayed grey. I was in monitor write mode.


But the missing I/O in the program baffled me. The inputs came on (led lit) and the machine did what it should but there was no reference in the program

To be exact, some inputs outputs were there from each station and I could see them working correctly.

I'm open to suggestions as to what these pc boards are doing.




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Check for reads and writes using ANY input or output.  What I mean by that is, do a search for X0 and see if there are any reads associated with it.  They may have programmed it to read in whole words of inputs at a time and write that to an internal word (D0, for instance).  Post the program and rack layout if you are allowed.  I currently do not have the software installed, but depending on the format, may be able to get it installed.  Regarding the PC boards, it is possible they are converting the signals from DC to AC or PNP to NPN?  If you have prints, post them as well (if you are allowed).

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What you may see is the address X1 may not be used, but perhaps you will find something like K4X0, which indicates 16 bits starting at X0.  It's possible that an entire word of inputs is referenced and moved somewhere using this notation.

K1X0 is 4 bits long starting at X0

K2X0 is 8 bits starting at X0

and so on...

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Thanks. I don't have the program with me but i'll be going back.

I did have a look for indirect addressing but it's a big program.

My suspicions are mounting that this is some kind of way to keep outsiders out.


I'll be back with more details later... with this lockdown, they are only working 2 days a week with skeleton staff


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