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libmodbus

A Modbus library for Linux, Mac OS X, FreeBSD, QNX and Win32
Not compliant device can be time consuming!
compliance , gnomefr

Travis Atkinson of Com-Pac Filtration and me have worked together to find out why the HMI of Weintek Labs I-Series, with Ethernet, answered only to request sent by the modpoll program (proprietary software).

After many emails and the help of Wireshark, we have discovered modpoll uses the unit identifier as slave ID for TCP requests, according to the Modbus specs that's incorrect (cf page 23 of Modbus Messaging Implementation Guide v1.0b) but else the device drops the request!

Two problems can sometimes lead to proper operation or Weintek company has designed this product on modpoll requests :-/

PS: The workaround was to set the slave ID with modbus_set_slave().