S5-135U going to STOP mode

I have an S5-135U. We had a problem in the past where if we unplugged our Siemens M4 PG while still in Windows but offline and out of STEP5. The PLC would intermittently go to STOP mode. I was told that it was a COMM port that wasn't completely "off" so to say. So we followed a principle of shutting down the PG and removing the charger, waiting one minute and then unplug from the CPU. This has worked great for 2 years but today it happened after follwoing the same process. It is one of our main PLC's and causes major downtime. Has anyone had this problem before or have a solution to ensure this does not happen? See below for PLC hardware.

Bottom Rack (Start of PLC)
Power Supply PS955, AC 230/120V, 5V/18A
Rechargeable Lithium Battery (RB) ACCU F. CC 188U AND EU 185U
Backup Data Battery (MB) BACKUP BATTERY LI, SIZE C, 3.6V/5.2AH
Central Rack CR135U/155U, 21 SLOTS, AC 120/230V/18A
CPU CPU928B CENTR.CONTR.MOD. F. S5-135U/155U
CPU RAM MEMORY SUBMOD. 377 RAM 64KB, SHORT TYPE
Communications Processor CP 525 COMMUNICATIONS PROCESSOR
Communications Processor EPROM MEMORY SUBMOD. 372, CMOS-EPROM, 32KB
Analog Input Module 466-3 ANALOG INPUT, 8/16AI, FLOAT.
Analog Input Module 466-3 ANALOG INPUT, 8/16AI, FLOAT.
Analog Input Module 466-3 ANALOG INPUT, 8/16AI, FLOAT.
Analog Input Module 466-3 ANALOG INPUT, 8/16AI, FLOAT.
Analog Output Module ANALOGAUSG. 470-4, 8AO, +-10V/0-20MA
Analog Output Module ANALOGAUSG. 470-4, 8AO, +-10V/0-20MA
Analog Output Module ANALOGAUSG. 470-4, 8AO, +-10V/0-20
Analog Output Module ANALOGAUSG. 470-4, 8AO, +-10V/0-20MA
Interface Module IM304 INTERFACE MOD., 600M
Connect Cable CONNECT.CABLE 721-0, 1M

Top Rack (End of PLC)
Power Supply PS955, AC 230/120V, 5V/18A
Backup Data Battery (MB) BACKUP BATTERY LI, SIZE C, 3.6V/5.2AH
Central Rack CR135U/155U, 21 SLOTS, AC 120/230V/18A
Interface Module IM 314 CR INTERFACE MOD. F. DISTR.CONN.
Terminating Resistor TERMIN.RESIST.CONN. IM314
Digital Input Module DE 430-4, 32E, DC 24V
Digital Input Module DE 430-4, 32E, DC 24V
Digital Input Module DE 430-4, 32E, DC 24V
Digital Input Module DE 430-4, 32E, DC 24V
Digital Input Module DE 430-4, 32E, DC 24V
Digital Input Module DE 430-4, 32E, DC 24V
Digital Input Module DE 430-4, 32E, DC 24V
Digital Output Module DA 451-4, 32A, DC 24V, 0,5A
Digital Output Module DA 451-4, 32A, DC 24V, 0,5A
Digital Output Module DA 451-4, 32A, DC 24V, 0,5A
Digital Output Module DA 451-4, 32A, DC 24V, 0,5A
Digital Output Module DA 451-4, 32A, DC 24V, 0,5A

McAGreg said:

It is one of our main PLC's and causes major downtime.

Click to expand...

I have never had that issue before but, you may want to start to consider replacing the PLC, its close to 25 years old and could be at the end of its life cycle

The behaviour sounds like something is electrically on the edge of failure.
It used to happen sometime under some strange condition, and now it happens more frequently. Dont wait any longer and dont count on finding some kind of "fix".

The most probable thing that goes bad first is the power supply. Getting any spare for an old S5-135U system will be very expensive, and will only give temporary relief.

Migrate the 135U to a modern PLC. It will cost less than buying spare parts and praying it will fix the problem.

First of all how do you re-start the PLC, is it possible to re-start by switching from stop to run ? or do you have to power off.
Can you go on-line once it has faulted to check the I & B Stack to see what error it reports. This may give you some idea to what causes it to go into stop. If the reported fault is random i.e. different at times then I suspect spikes down the coms when unplugging it. however there is another possibility that the scan time of the processor is close to the max setting (common on 135 as the processor is slower but expected to give same performance as 155). the unplugging may cause the processor to be interrupted causing a scan time issue. I once had a similar problem on a 135, when the PG was connected it would sometimes go into stop, this turned out to be scan time setting, we did not want to increase the scan time as there were some critical processes that we felt would be affected, we managed to remove some redundant code where a part of an independent process was removed some years before & this cured the problem. To make sure we modified the code therefore taking out many instructions to improve scan time Just a thought. But I do agree with the upgrade as replacement parts are expensive or may not be available. S5 was a good system, however on another project a 150 was replaced by the newer 115 (945 processor) the system had 4 coms cards H1 & 525's when replaced the coms cards would not talk on cold start i.e. power up, we got round this by putting large loops to hold it in the start up OB's this worked a treat.

I just switch to stop and back to run.
This is the first time in two years and the machine is production based, so getting back online quickly was my main concern. I am hoping to try and recreate this on a shut. I will then have time to check the I & B Stack. I do not believe the scan time is anywhere neatr max setting as you say. How can i check this?

An upgrade would be a dream come true for me.

Try using an optical isolator on the RS-232 side.

https://lmgtfy.com/?q=rs232+db9+optical+isolator&s=g

Edit: Sorry, the above link will not apply if you are using on-board 20ma/TTY direct from the Siemens PG port.