Modbus Decode

[drbitboy]

That is for Modbus Function code (command) [0x05 Write Single Coil]?

Is that documented anywhere?

 

[TheWaterboy]

I asked that very same question and I'm waiting for his reply.

 

[drbitboy]

I asked that very same question and I'm waiting for his reply.

I am not seeing anything about it in the micro8xx help docs.


With the 5380 I assume you are using the AOI from TechConnect, so Caveat Emptor, eh?

 

[TheWaterboy]

Support is not reading my question and is replying with boilerplate. I hate it when the chat based folks are the only ones there.

 

[TheWaterboy]

Well RA asserts that in the entire history of Modbus serial no one ever has used FF00 to set a coil. It's always been done by a boolean in that address and then translated within the device doing the sending to FF00.

It's what works in this AOI so I guess its true.

 

[Steve Bailey]

I agree it sounds like a strange method to set a single coil, but that's what the Modbus spec says to do. Perhaps because 'under the hood' the Modbus server's response to command code 05 is to read a byte from memory, modify the desired bit within that byte and write the full byte back to memory.

Well RA asserts that in the entire history of Modbus serial no one ever has used FF00 to set a coil. It's always been done by a boolean in that address and then translated within the device doing the sending to FF00.

It sounds like that's the way Rockwell has always done it. And of course there are two ways to do anything, Rockwell's way and the wrong way, correct?

 

[TheWaterboy]

I'm sure they just use the last bit anyway. If I was so far off base I would have expected Ken Roach to come here and hammer me about this. Love Ya Ken!

Just to make sure this horse is deceased... If they wanted to simplify something they could have encapsulated the registers into the desired data structure so all the bit banging would no longer be necessary! Essentially it's the same thing carried to its logical conclusion.

Bah I'm done. See ya