Endress+Hauser EngyVolt RV12, Руководство по эксплуатации

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Appendix EngyVolt RV12
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11.4.1 Modbus data format
The Modbus Protocol defines the format for the master’s query and the slave’s response.
The query contains the device (or broadcast) address, a function code defining the
requested action, any data 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.