This simulator was originally created around 1999 to allow testing of a driver for a Texas Instruments 500 series controller which I had limited access to while writing the driver. A year latter I re-wrote it to support modbus RTU and TCP, basically because the simulator had allowed me not only to learn the role of master, but also of slave. So that others could benefit from the kind of test I was able to set up, I published the modbus simulator on the web in 2002. It may be used as a "virtual" PLC for training and easily simulating for test or development purposes. This simulator was written to allow the testing of a MODBUS RTU serial driver without having to get 254 little MODBUS PLCs into one office. As such it fulfills the need very well, but is not, by any means, a full implementation of the protocol; it implements only the common functions used in MODBUS and other protocols supported here.
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