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are you going to develop a polling program for pc? or are you going to develop device that equip with modbus protocol?
for the first one is eazy, basically you can try modbus ocx from wi**ercom. (*=t) its eazy. or if you know how to write comm. program, then you only need to refer to modbus protocol manual, can be downloaded from www.modbus.org. and use the appropriote function in you application. i'm only use two fucntion, a poll and write function.
if you need to develop embedded device with modbus, i think these is some difficulty. there is some chip equip with modbus protocol. if you need to design it from baginning, then you need to refer to the standard.
Actually, Modbus is a very simple protocol to implement once you get past the 3.5 character delay timers. In an embedded system, it is even simpler since you have direct control over your timers.
Are you designing a Master or a Slave? Accordingly, the complexity changes.
As wisely77 suggested, www.modbus.org has the complete manual. I do not agree with using only two functions. If you are writing something that should communicate with a generic Modbus slave, then you should support all the functions since many slaves support different types of read and write functions. Problem is mostly with the write functions since Modbus supports two different types of write functions.
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