Appendix
EngyVolt RV15
46
to be sent, and an error-checking field. The response contains fields confirming the action taken,
any data to be returned, and an error-checking field. If an error occurred in receipt of the message
then the message is ignored, if the slave is unable to perform the requested action, then it will
construct an error message and send it as its response. The Modbus Protocol functions used by the
EngyVolt devices copy 16 bit register values between master and slaves. However, the data used by
the EngyVolt device is in 32 bit IEEE 754 floating point format. Thus each instrument parameter is
conceptually held in two adjacent Modbus Protocol registers.
Query
The following example illustrates a request for a single floating point parameter i.e. two 16-bit
Modbus Protocol Registers.
First Byte
Last Byte
"Slave"
address
Function
code
Start address
(Hi)
Start address
(Lo)
Number of
points (Hi)
Number of
points (Lo)
Error
check (Lo)
Error check
(Hi)
"Slave" address:
8-bit value representing the slave being addressed (1 to 247), 0 is reserved for the broadcast address.
The EngyVolt devices do not support the broadcast address.
Function code:
8-bit value telling the addressed slave what action is to be performed. (3, 4, 8 or 16 are valid)
Start address (Hi):
The top (most significant) eight bits of a 16-bit number specifying the start address of the data being
requested.
Start address (Lo):
The bottom (least significant) eight bits of a 16-bit number specifying the start address of the data
being requested. As registers are used in pairs and start at zero, then this must be an even number.
Number of points (Hi):
The top (most significant) eight bits of a 16-bit number specifying the number of registers being
requested.
Number of points (Lo):
The bottom (least significant) eight bits of a 16-bit number specifying the number of registers being
requested. As registers are used in pairs, then this must be an even number.
Error check (Lo):
The bottom (least significant) eight bits of a 16-bit number representing the error check value.
Error check (Hi):
The top (most significant) eight bits of a 16-bit number representing the error check value.
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