Communicating PLCS with Radio modems (Modbus protocol)

[Jesus Baez]

Dear Sirs

I am trrying to communicate two PLC`s (slave-master configuration) via radio modems using RS485 ports. Using ?native? protocol I can communicate both PLC?s. However, using modbus RTU or ASCII I am not able to communicate them.

I performed another test in which a PC reads information from a ?remote ? PLC via radio modems connected to their RS232 ports ,and it works fine.

I think this problem has to do  with timing. Is there any way in which master PLC  allows more time to receive response from slave PLC when using Modbus ASCII/RTU?

On the other hand, I have a PLC with a Xserver attached to it and I am able to send information to a fixed IP address PC running Webserver. It works fine when Xserver is connected to my LAN which is connected to Internet via a DSL provider. I recently installed a satellite internet service equipment (for remote telemetry applications) and I am not able to transmit information using PLC+Xserver. I talked to ISP and he assured that all TCP/IP ports are enabled and open. When I connect a PC to the modem (satellite internet) I am able to access websites, send emails and doing ftp at a very reasonable speed.

Do you have any idea of what the problem might be and how can be solved?

Your help will be greatly appreciated,

Sincerely,

  Jes?s  Baez


 

[support]

Please see help file for "SETSYSTEM" command. I believe SETSYSTEM 1,2 or SETSYSTEM 1,3 will probably resolve the issue if it is indeed due to waiting time for a response from the slave. I am a bit surprised that NETCMD$ works but not READMODBUS since they actually use the same default waiting
time of 0.15s.

Are you able to connect to your XServer using TRiLOGI program when the XServer is connected to the Satellite Internet? Are you connecting to it directly or via a NAT router? Do you need to login to the account (PPPoE) if it is connected directly to the Satellite internet? If so then you should get a NAT router to do the login to the Satellite modem and the XServer can connect to the router via normal ethernet without login.