Auto 485 - disconnection problems

[ccdubs]

Hi all,

I use an Auto 485 to connect to my T100MD888 plc for downloading and monitoring purposes while at the same time I connect my PC to the RS232 com port of the PLC for MODBUS comms generated by DAQFactory.

This is allows me to online monitor while the PC and PLC comms are active, which is great for development and debugging. Unfortunately the 485 coms are sporadic at best and while I can connect and monitor, the monitoring updates are not continuous and I will lose connection typically in < 10s or the connection freezes and takes 10s plus to re-establish.

This happens even when the PC is not sending MODBUS requests, interestingly however I always seem to be able to transfer programs through 485 without any problems.

FYI, as the PC has no comm ports I am using a usb-232 converters for both channels.

Any ideas what the problem may be?

[support]

A normal system should not have such a problem. We frequently run both COMM port simultaneously with both as slaves (as per your case) or one as slave and the other as master or both as masters and they work well. However, please ensure that your program is not sending out any data out of COMM3 as that would interfere with the communication with TLServer.

A quick test to see if it is the software problem or if the Auto485 is working properly:

Transfer a blank program to the PLC.

Then only connect the PC via Auto485 to the COMM3 port and perform online monitoring using TRiLOGI and TLServer. You should get good connection that can run for many hours.

If you still see frequent problem with a blank program, then the RS485 driver on the Auto485 may be slightly damaged and that need to be replaced.

[ccdubs]

It appears that it must be the Auto485 as the comms still failed even with a blank program.

I will try again with a direct serial connection from my PC to the auto485 when I load TLserver onto it, just in case the problem is coming from the USB to serial converters (I have had issues with these in the past).

Will let you know.

[support]

You mentioned that you are able to use the USB-RS232 converter to transfer  program to the PLC via RS232. Are you able to continuously monitor the PLC via RS232 port this same USB-RS232 converter? If so, then the USB-RS232 convert is probably compatible.

Next to check or replace would be the RS485 drivers on the PLC as well as on the Auto485. These are the 8-pin device with one of the following part number:

1) 75176 (those before 2007)
2) 75HVD3082 or 65HVD3082
3) LT1785 (high voltage fault-tolerant version).

You can try to replace one of the above chip with a new one and check if the problem would go away.

[ccdubs]

It would appear that it is the Auto485 as I was able to monitor with the same USB converter on com1. Interestingly though the MODBUS coms through the Auto485 still worked but there is no easy way for me to determine how reliably.

This isn't the first time I have had this problem so I will focus on the Auto485 as the problem.

[support]

There is a little preset potentiometer on the Auto485 that you can try to turn to change the crossover timing. It may help a bit but if the problem is due to RS485 driver then it won't make much of a difference.

Looks like you may have narrowed down to the problem being the RS485 chip either on the PLC or on the Auto485 adapter.

I presume that the Auto485 adapter and the PLC are powered from the same power supply? If not, then you should connect the 0V terminal of the PLC power supply to that of the Auto485' 0V terminal to provide a common ground reference.