Multitek MultiLed M812-LD Series Operating Manual page 51

Low level character format :
With Odd or Even parity = 1 start bit, 8 data bits, 1 parity bit, 1 stop bit
With NO parity = 1 start bit , 8 data bits, 2 stops bits.
5.1.3 Modbus Format
Data format :
RTU
32 bit, floating point format to IEEE-754 standard, exponent bias 127,
most significant byte transmitted first. There is no word reversal.
Address range: 1-247
Error Check :
Cyclic Redundancy Check (CRC), Polynomial exclusive OR value
A001h Low byte sent first.
5.3.4 Modbus Protocol
Only the RTU (Remote Terminal Unit) form of the Modbus protocol
will be supported and all references in this sheet refer to RTU format.
Typically, the master or controller requests data from its slaves by
polling them one at a time and waiting for a pre-defined amount of
time for a response. If a node doesn't respond within this time period
the master will try accessing it a further two times before indicating to
the Scada system that the node has gone off-line or there is a fault
with it. If all is well the node will respond with the data requested.
A typical communication 'packet' will be in the form :
[Node Address] [Function] [Byte count, Data etc.] [Error check].
Because Modbus is a word (2 byte) orientated protocol, floating point
(4 byte) register numbers are counted in multiples of two.
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