Unique PLC Problem

  • Thread starter Tribhuwan Joshi
  • Start date

Thread Starter

Tribhuwan Joshi

I am facing this abnormal behavior in a Pump and Cylinder manufacturing company.
The connected PLC is Siemens S7-200 i.e CPU 216 with three numbers of EM 223.
This is controlling the cutter machine for different jobs.
The problem is that sometimes, after power up the total Panel with PLC , without giving any desired input it starts generating the output. After seeing this the supervisor cuts the power and start making some knocks on the outboard of the modules.
After knocking he again powers up the total panel and now it works perfectly.

Martin Medina


i used the S7200 plc some years ago and is a very bad device the problem is the kind of board conector, this conector generate false contacts an false output activation. The solution is to clean and be sure of the conections between the board and the conector

I did change the Siemens S7200 for an Omron C200H.

Good Luck

Martin Medina

Jeff LeBlanc

This could be cause by a loose ground or loose input signal wire. I suggest checking all terminal blocks/terminations in the cabinet.


Marc Sinclair

Assuming, that the program is good :) and that all other connections are ok look at the PLC system, The series one modules, were interconnected by black edge 'clips' directly onto the PCB. Before you connect the clips, you need to break small covers over the edge connectors, these covers sometimes left some plastics 'legs' which interfered with the connection, ensure that all the openings are clean and clear. (The series two PLC - 222, 224 & 226 - all use ribbon cables.) Also , check that the modules are clipped securely to the DIN rail, I used to use a DIN clamp at either end of the system. Let me know what happens

Marc Sinclair

Michael Griffin

This may possibly be a loose connection in the bus between the CPU and the I/O module. The S7-21x series used an edge card type connection to the I/O modules. I believe that Siemens had enough problems with this that on the next generation of product (the S7-22x series) they changed this to a ribbon cable. I haven't had any such problems myself, but I was told this was the reason for the change in connection design.

If the modules are DIN rail mounted, you may check if they are securely held together with end stops on the DIN rail. You may also try changing the first module in the chain which has this problem.

If the problem persists despite fiddling with it and you decide to change the whole PLC (they are quite inexpensive), use one of the new series as the bus connection system is more secure.

Michael Griffin
London, Ont. Canada ************************

Hakan Ozevin

This behaviour is already written in S7-200 manual.

It is mentioned that, after a power up, it is possible that an ON signal can be sent to the outputs for 50 msecs (page 347 of S7-200 manual).

The remedy is simple: Feed the outputs with a separate power supply which take the enable signal from the CPU.