Modbus tcp read holding registers not returning requested quantity

Question

Background:  I have a customer using an ELAU motion system - they are recording data to Holding Registers that they want to be able to read on a cRIO to match up with some analog data from FPGA (I have digital handshaking happening for that).
LabVIEW 2010 SP1
cRIO 9074
Using the NI MOdbus.llb VI Library to communicate with the other system.  I can open the TCP communication no problem, and actually get SOME registers, just not ALL of the register data I want to read. 
What I want to do is read 330 individual holding registers U16 values.  I know how the data is being split to represent various lengths (i.e. most data items are using 2 registers to represent one 32 bit number).  I just want to read all the individual registers and parse the data in another VI to convert it to the other data types.
I provide the MB Ethernet Master Query (poly).vi with the starting address for the first register and then the quantity of 330.  The polymorphic instance selected the is "Read Holding Registers".  The array returned from this VI via "Holding Registers" is only 74 elements and not 330 requested.  I get no exception codes and no LV errors.  Is there some inherent limit on the number of Holding Registers that can be read?
I am not using the MBAP Header input (not sure if that is needed).
Thank you.
Ryan Vallieu
Automation System Architect
Solved!
Go to Solution.

Answer 1

Simple solution once I dug into the MODBUS Serial protocol/TCP protocol documentation available out there via Google.
Serial based modbus history is the limitation that carried over to TCP - the maximum amount of bytes in one data pack can only be 256 bytes.  Thus I was limited to around ~125 registers at once.
256 bytes = 2048 bits.  Using 16 bit registers that gives 128 registers maximum.  I went with 125 to make keeping track of totals easier.
Ryan Vallieu
Automation System Architect