How to determine comm timing

I'm designing a system which will have up to 9 D06's communicating on a Modbus network using port 2. I'll probably be gathering 20 registers from each unit. How do I determine the communications delay to acquire the 20 registers from all 8 units. The ninth unit is the master.

Thanks,
Jim

The overhead on modbus is roughly 11 bytes (that includes the 3 byte time delay required by the protocol). You'll be issuing a read command (12 bytes) and waiting for a response of 11 + 20 bytes (if you use block reads. You're up to 12 + 11 + 20 = 43 bytes per node * 8 nodes = 328 bytes. 344 bytes * 11 bits per byte (start, 8 data, parity, stop) = 3784 bits total.

Divide by your baud rate (bits per second), and that should be your magic number.

AK

Thanks AK, that's exactly what I was looking for.

It's been close enough for me, in most cases.

I just noticed that I corrected one of my total byte counts and not the other. I rechecked the modbus spec and had to increase the bytes per node. The right number is 344.

AK