Thread: Velocio PLCs?
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Old December 14th, 2017, 06:46 PM   #9
United States

Brandon_K is offline
Join Date: Mar 2016
Location: Pittsburgh, PA
Posts: 139
Originally Posted by AustralIan View Post
Modbus doesn't have address 0, the first register is register 1. It is however transmitted as 0x00 if you look at the packet that is sent. When commissioning a new modbus device, I always check to see if their addressing is offset by 1, because many manufacturers refer to the transmitted address and not the modbus standard address.
That's a great explanation and now I understand.

The issue cropped up when I had a program written to intercept modbus requests via a RS232 adapter and transform them into HTTP status messages (as the vendors software could only poll HTTP requests). That explains why they were looking for the hex data instead of the addressing that I'm accustomed to. It took a while to figure out that their bridge program was looking for an offset of -1 vs my addressing.

Thanks for the info.
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