Hello,

I have problems of communications between remote Modbus stations over GPRS TCP/IP with Premium PLC.

I use READ_VAR to read Modbus Registers, but in two or three days, PLC Premium stops communicating.

I need to communicate with 9 remote stations over GPRS TCP/IP.

How I must program the PLC?

Thank you.

 

After the system stops communicating, can you get it to start working again by restarting the master?... or the slaves?

Your answer will give the forum an idea of whether the problem relates to the device(s) reaching their limit of open sockets.

 

when the Schneider Premium (Master) stops communicating, if I restarting the Premium, it work again.

The slaves always communication, if I use ModbusPoll.

The problem I have is in Premium.

Thank you.

 

Since the problem is at the master, I'd suggest inserting a hub (an old switch) into your working network. Connect the master premium PLC and a PC/notebook to the hub and run Wireshark on the PC/notebook to snoop the Ethernet traffic to see what is happening.

Wireshark has the ability to decipher Modbus packets, so it should give you plenty of clues.

Hope this helps!

 

Hello,

Does the communication hang between Premium and GPRS modem or communication over GPRS? Maybe your APN from time to time disconnect the link and you do not have procedure to re-initialize the GPRS modem.

Regards,
Andrzej
www.micro-tech.com.pl

 

I'd also recommend plugging the GPRS modem into the same hub that I mentioned above... so that its LAN traffic will also be visible to Wireshark.

 

> I use READ_VAR to read Modbus Registers, but in two or three days, PLC Premium stops communicating. <

I would use the PLC to power cycle the GPRS modem daily (or if it stops). Here are two common bad things which can happen on cellular:

1) A 'good-signal' cell tower goes off-line, so the modem moves to a 'bad-signal' one. Even after the 'good-signal' tower is back on line, the modem will NOT move back so you have horrible performance until either the 'bad-signal' tower goes off-line or the modem is power cycles. This is especially a problem if you use directional/yagi antenna.

2) 'Negotiation' of who pays the data bill can be complex when carriers 'share' towers. In some cases your data is moving through 3 or more companies. If any one of those companies forgets after some equipment reset that you are a paying customer, your PPP/IP connection goes 'quiet' - it MAY NOT be perceived as broken even though it is. So the cell modem thinks you are connected, but your are not.

In our cellular products we have tools which 'soft reset' PPP links quiet too long, plus tools to actually power-cycle the cell modem if the PPP link is still quiet for much longer periods even after PPP resets. Your PLC hard resetting the modem serves the same purpose. You want to force a complete renegotiation with the cell system.

regards
- Lynn August Linse, Industrial Specialist, Digi

 

I don´t have any problem witch GPRS VPN network. When Premium stops comunications, GPRS VPN work OK.

In this case, if I have a PING command to any station, it responds OK

The problem that I have is in Premium programming.

Thanks.