Internet PLC Forum

Hi, I have had to swap an old T100MD888 with a new FMD88 but had an issue with the analogue input values I was reading. From what I can see from the connection wiring the only difference is that our application connects the future D/A's #3 and #4(pins 10 and 11)  to 0V as that is what these pins were on the T100MD888.

Would this be the cause of the spurious analogue input readings I now get? If not, are there other differences (software and hardware) that I should be made aware of when swapping a MD888 with a FMD88?

Thanks.

I have had a situation with an FMD88 where I could still communicate via ethernet and monitor the status of inputs but when I tried to activate outputs through my SCADA interface, the physical outputs didn't turn on but the status read back from the PLC showed that they were activate. This would imply that the code was still functioning as there is logic that maps SCADA commands to outputs but the physical outputs didn't trigger.

I re-powered the PLC multiple times and it made no difference. After leaving the PLC off for 30 minutes or so and turning back on again, it seems to be functioning correctly. I have not been able to replicate the issue since.

I can't say for sure but I don't think the RTC Error light was on while I had the problem. Any ideas?

Hello,

We have a number of T100MD888+ installed in remote sites that communicate over a VHF radio network. We have always noticed at some sites that there has been some interference to the radio that we have always considered as being from an external source. Recently this level of interference has become problematic and we have employed RF consultants to investigate the issue. The result is that the interference is coming from the T100MD888+ PLC and more accurately their "RF sniffer" identified the noise emanating from the CPU.

Our radios work in the VHF spectrum at around 150MHz. I was originally skeptical to the PLC being the issue but based on their analysis, I tend to agree (i.e., removing power from PLC stops the source of interference).

Are you aware of this product generating RF interference? Do you have any recommendations as to how we could reduce this interference? It is unusual that it would seem that things have degraded over time (although that could be explained by other influences) but is there anything within the PLC that may degrade and cause RF noise?

Any help, as always, is greatly appreciated.

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?

I have started using the FMD88 PLC for the first time and have experienced some anomalies with the analogue inputs. After some fault finding, it looks like some of the channels may be using a different averaging constant. I indentified this as I need to sample AI channels only once after an event where the AI value from the sensor is latched, read by the PLC and then cleared back to 0. When the PLC reads the value I noticed that the values of ADC 4 and 6 were half of ADC3 and 5 respectively when they should be the same and the voltage in to the PLC was correct.

I think this can be explained by the averaging constant being set. This PLC is fresh out of the box and after looking at the AI calibration it looks like values the ADC Calib and the avg constant have been set. What should these settings be when shipped from the factory?

It doesn't look like the averaging constant can be set per channel so I can't understand why some readings are correct and others aren't.

Is it possible to use Modbus TCP on a F-series PLC as a slave and use send commande using Modbus broadcast mode from the master?

I have a strange thing happening where randomly, the T100 PLC will stop communicating with a PC set up as the MODBUS master.

Rebooting the PLC has no effect. If I depower, switch DIP4 on, repower, switch DIP4 off, communications work again.

Does this make any sense?