Q2167 How to Disable Multiple Coil Writes to a Modbus Device

ArticleID: Q2167
Created: 7/07/1997
Last Updated: 23/11/2007
Suggested By: Alex Armitage

Applies To:

CitectSCADA 3.xx, 4.xx, 5.00

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Summary:

I need to communicate to a Modbus device using Citect's Modbus driver. However the device does not support multiple coil writes (Modbus function code 15). I get the error message "File (y:\pc\source\IOServer\PLC_Drv.c) Line (1769) Error Req Size Too big 16" at runtime. How can I disable the blocking of writes to my device?

Solution:

Set the parameter [IOServer]BlockWrites=0. This parameter will disable all blocking of writes to all protocol drivers. This will stop the Modbus driver from using command 15 (write to multi bits), and will use the command 5 (write a single bit out) instead.

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