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Modbus does not activate in connected GSM modem

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I can not activate modbus in V570 via GSM. I am using same software routines that work fine with V120. Modem initialises fine in 

COM 1 (sb80=1), modem also connects fine when connected/called from modbus polling software (SB 86=1).


However, the modbus traffic doesn't initiate. The polling software says "Connected" and then "Modbus request failed".


Modem status in PLC (SI 80) lays with value 2. When connected, the value is 4, then chages to 1, then back to 2.


The polling software should be fine, it activates modbus OK with V120. So there may be some software problem in my V570 ladder code,

even it works in V120. Same modems and cables are used and V570 com port dips are in RS232.


What can prevent modbus traffic from being activated. I have used the Modbus config, SCAN, SCAN_EX and Porotocol config

function blocks but looks like I must be doing something wrong - even they work with V120.


All ideas greatly appreciated!


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