Fmd88 outputs toggle at power on

[ccdubs]

I have noticed with the fmd88 that outputs 7 and 8 briefly turn on when the plc is powered up. For my application this briefly toggles a heater relay so no harm done but for other applications this could be a significant hazard.

Is this a known issue?

[support]

These are the transient voltage spike appearing on the I/O pins during power up which cannot be totally eliminated. Output 7 & 8 being high speed MOSFET respond faster than the other Darlington transistor output which is why it is briefly visible. The voltage spike is not sufficient to turn on any kind of load. But if the output is connected to devices that respond to slightest input signal spike then you could use a relay to isolate the signal.

[ccdubs]

My heater is swiched by a 12v omron relay, this relay momentarily switches on startup. It was this relay switching that drew my attention to the brief on/off of outputs 7 and 8. So it would seem that there is enough transient voltage to switch a relay.

[support]

Are your PLC and your output relay powered by the same 12V power source? If not, how are they connected?

There is a pull down resistor at the gate of the output driver chip that keeps the driver OFF before the CPU is powered up properly to drive the output to completely off. In our test the pull down action is sufficient to keep the output off even though in some situation there may be a brief spike when power up due to the time constant but that should not be enough energy to turn on a relay. Otherwise you might have a faulty driver chip which may need to be replaced. Please contact tech support for RMA if this cannot be resolved in the setup.

[ccdubs]

Yes, the heater relay and PLC are connected to the same 12V source.

When I noticed the problem I had 22 PLC's that were being commissioned in our product. While I can't conclusively say that this was happening to all PLC's it definitely wasn't unique to just one and would have applied to most.

The relay used is an OMRON G2RV-SL700 DC12.

[support]

We have checked with the factory and noted that a recent change of the resistance of the pull down resistor has resulted in this issue of the driver not being disabled quickly enough during power ON (i.e. before the CPU is ready to control its output). The output 7 & 8 driver are therefore briefly enabled during power up. This is indeed undesirable and we have escalated this issue to the design/manufacturing team. We have received confirmation that the factory will be reworking boards in our stock that are affected by this issue. You mentioned in your earlier post that your system would not be affected by this issue so no action is necessary. However users whose system cannot tolerate this brief spike can contact tech support to get a RMA for a free rework to fix the issue.

One recommendation for safety-critical system is to supply the +24V to power to the load via a 1 second or longer on-delay timer so that the output power are only enabled when the CPU is fully powered up and in full control of the output driver.

[ccdubs]

If I understand correctly, the CPU needs to be running to keep the output off. This makes me wonder what failure modes there may be that could make an output turn on when not wanted. I.e., if the CPU were to fail, would outputs 7 and 8 turn on? What about outputs 1-6?

Are there any other failure modes that could cause an output to turn on?

[support]

The PLC circuitry is designed to hold all outputs to OFF state in the case if the CPU is brain-dead (i.e. does not start up at all after applying power). The output 7 & 8 are also to be put into OFF state by the pull down resistors attached to the driver input pins. Due to the ultra fast switching speed of the MOSFET drivers on output 7&8 lower pull down resistor values are needed so that the time constant to hold the output to OFF state are shorter than the rise time of the CPU output pin to +5V in order to hold the output to completely off when power is first applied to the CPU.