I am currently slaving away at some Mach4 (and some paying work for another project), so, time I don't have for a plugin.
BUT: Look in the back of the GS3 Manual, and look at the Modbus addressing sections, for the Drives parameters.
BTW: I HIGHLY recommend you download and use the FREE GS Motor software from ADC, to set up your motor.
At any rate, in the "Users Documents" area, download and read my "Brains" document, it will show you how to talk to stuff like a VFD/PLC through
modbus, you really only need a FEW areas of the drives modbus registers: On/Off, Cw/CCW, RPM, and possibly if you want them, Enable/Disable, Drive Fault output. You can use "Group" parameters in the drive, to put all the above in ONE continuous block for addressing, then set up your Mach Modbus "Pluigin Serial" to that block of addresses. Then Use Brains to read/write to that block your commands.
Scott