84
Data is encoded for each character as binary data,
transmitted LSB first.
For a QUERY the address field contains the address
of the slave destination. The slave address is given to-
gether with the Function and Data fields by the Appli-
cation layer. The CRC is generated from the address,
function and data characters.
For a RESPONSE the address field contains the ad-
dress of the responding slave. The Function and Data
fields are generated by the slave application. The CRC
is generated from the address, function and data char-
acters.
The standard MODBUS RTU CRC-16 calculation em-
ploying the polynomial 2
16
+2
15
+2
2
+1 is used.
Inter-
message
Gap
Address 1
Character
Function 1
Character
Data n
Characters
CRC
Check 2
Characters
Supported Modbus Functions
The following Modbus function types are supported by
this instrument:
Function
Code
Modbus
Meaning
Description
03 / 04
Read
Holding/Input
registers
Read current binary
value of specified num-
ber of parameters at
given address. Up to 64
parameters can be ac-
cessed with one query.
06
Write Single
Register
Writes two bytes to a
specified word address.
08
Diagnostics
Used for loopback test
only.
16
(0x10 hex)
Write
Multiple
Registers
Writes up to 253 bytes
of data to the specified
address range.
23
(0x17 hex) Read/Write
Multiple
Registers
Reads and Writes
253 bytes of data to
the specified address
ranges.
Function Descriptions
Function 03 / 04 - Read Holding/Input Registers
Reads current binary value of data at the specified word addresses.
QUERY: Function 03 / 04 - Read Holding/Input Registers
Func
Code
Address of
First Word
Number of
Words
03/04
LO
LO
HI
LO
QUERY: Function 03 / 04 - Read Holding/Input Registers
Func
Code
Bty
Code
1st Word
Etc.
Last Word
03/04
XX
HI
LO
g
HI
LO
Note:
In the response the “Number of Bytes” indicates the number of data bytes read from the instrument.
E.g. if 5 words are read, the count will be 10 (0xA hex). The maximum number of words that can be read is
64. If a parameter does not exist at one of the addresses read, a value of 0000h is returned for that word.
The following is interpreted from the Modbus protocol
description obtainable from www.modbus.org. Refer to
that document if clarification is required. In the func-
tion descriptions below, the preceding device address
value is assumed, as is the correctly formed two-byte
CRC value at the end of the QUERY and RESPONSE
frames.
Summary of Contents for Chromalox 4081
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