RAM Failure

[ccdubs]

I had a T100MD PLC fail today on site. Upon inspection the only noticible thing was that RTC Error light was OFF. A reset of the PLC didn't fix the issue.

I swapped the RAM chip with a spare I had and the PLC appears as good as new. I'm interested to know what could have caused the RAM chip to fail, as I have quite a few of these PLC's in operation. It has been in service for about 3 years.

[support]

It is hard to say. It could be static discharge, high energy short voltage surge coming in through power supply, or just a statistical component failure. Although this rarely happens. What is the brand or part number of of the RAM chip in use?  Is this related to the Modbus issue you reported in the previous post?

[ccdubs]

The chip that was faulty is a CY62256.

This PLC is completely unrelated to the ones in the other post.

[ccdubs]

Something similar has happened again, although I don't think the issue is the same.

I have had a T100MD-2424 returned and upon power up the green RTC light is OFF. So I tried my normal fix and replaced the RAM chip, but this didn't correct the problem.

What other issues would cause the RTC light to not go on.

[support]

T100MD2424 requires a 24V power connected to the Vp terminal in additional to the normal power terminal in order to enable the outputs (including the LEDs). Please check that if this has been connected. When you power up the PLC did you see that the 3 status LEDs (1 green and 2 reds) power up for 1 second or so and then goes off? If the PLC does not have MX-RTC then the green RTC.Err light will remain on after the initial sign-on. If there is MX-RTC then the green RTC.Err light will also go off after initial sign-on.

[ccdubs]

Yes the Vp terminal is connected to 24 Vdc.

The three lights do not turn on and there is no MX-RTC and the RTC.err light stays off

[support]

1) Is there any communication with the PLC via RS232 or RS485 port?

2) If there is communication but no status light, it means the output driver 74HC595 that controls the status light may be damaged.

If there is no communication at all that means something more serious may have happened and we recommend you request an RMA and send the products back to us for diagnosis.

[ccdubs]

I haven't been able to communicate with the PLC at all.

I did notice that when powering up, the output LED's partially light up for an instant.

I won't bother sending it to you, upon further investigation I believe the PLC has been mistreated and there is a good chance that it may have been shorted. The customer will have to buy a new one.

[ccdubs]

To continue with this thread and the fault reported in the January posting, I have had the PLC fail again in the same way. There is also another unit on the same network which is almost as problematic with the same failure mode.

The only relationship between these two units is that they both experience high levels of noise on the RF radio network as they are located close to boating marinas with a lot of UHF and VHF radio use.

I have found out that the radios we use can confuse noise with a transmission and will raise pin 1 of the com port high while the noise is there. Is there any chance that this could cause the failures we are seeing? What about false signals on any other of the com pins that aren't used? Only pins 2,3 and 5 are needed to communicate between PLC and Radio but all of the other pins are connected straight through.

[support]

electromagnetic noise may disrupt communication temporarily but shouldn't cause permanent failure of component.

What's more important is the power source into the PLC - what kind of power supply? Is this on some generator and battery arrangement? Is there a possibility of lightning strike affecting the equipment? You may want to consider adding surge arrestor or trasnsient absorber to the protect the PLC input power supply source.

[ccdubs]

The 12 Vdc PLC power supply is fed from a 24-12V DC-DC converter which has it's 24 Vdc side connected to a battery bank which is charged by the mains via a 24Vdc battery charging power supply.

So the PLC is well protected behind two DC power supplies and a battery bank in the middle to provide some surge tolerance.

There is very little chance of lightning strike in this area and we have multiple other units that don't display this same failure mode.

In the end a PLC reset seemed to fix the problem, if it was power supply damage, would this have corrected the fault?

[support]

Your earlier posts suggested some permanent damage (i.e. replacing RAM chip and still did not work) to the board. Your last post said a power on reset has fixed the problem - it means there is no physical damage if it can be fixed by a reset.

Actually battery bank that is connected to the main for charging can be subject to "load dump" when the battery is connected/disconnected from the charger.  There were reports that battery charging load dump factor could corrupt or damage the T100MD EEPROM. Surge suppressor should be installed to filter out high energy transient from the load dump effect.