Modbus Timeout after adding 3rd device?

[JatCosta]

Hey AzeoTech Folks,

I am running into a Modbus illegal address and timeout warning after I add more than two devices:

 

When I have only 2 devices it is fine...  Any Ideas?

 

[AzeoTech]

Illegal data address comes from the device and is a Modbus error code, not DAQFactory.  This can happen in one of several ways:

1) you are requesting an address that doesn't exist
2) you are using the wrong function (i.e. reading a holding register when in fact it is an Input register)
3) you are off by 1 register on a 32 bit value (U32, S32, float).  Remember, 32 bit values require 2 modbus registers, so you'll see in your docs that the addresses jump by 2's.  If it says 2, 4, 6, etc. and you try and read 1 or 3, then often the device will send an illegal address error because in order to read address 1, you really have to read 0 as well and combine them.
4) on that same note, you try and read a 32 bit value, but only request one register (because the I/O type is set to a 16 version).
5) you are using 0 based tag #'s for one device and 40,001 based tag numbers for another.  You have to choose which and stick with it.  I personally always use 0 based to avoid any confusion and because that is what is in the low level Modbus request anyway.  40,001 just gets converted to 0 before going down the wire to the device.  The whole 40,000 and +1 part is just fluff added many years ago when they thought it would be confusing for people to count from 0.