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User's Manual
9. Modbus communication
The CD3000M is equipped with two-wire RS485-compatible serial communications, by
which means communication may occur between the Controller and a master device
(e.g. a computer or terminal).
9.1 Physical requirements
9.1.1 Character Transmission
Data format is fixed to be one start bit, eight data bits, one stop bit, baud rate 9600 and the parity
none.
9.2 ModBus RTU protocol
The standard RS485 Communications uses the industry standard MODBUS RTU protocol.
The following restrictions are imposed:
Baud rates is fixed to 9600 Baud only.
•
Support for multi-parameter Write operations is limited to support of the Multi-
•
Word Write Function (Number 16) but it permits the writing of only one
parameter per message.
The following MODBUS functions are supported:
Function
Read Holding Registers (Read n Word)
Preset Multiple Registers (Write n Word)
The Controller will identify itself in response to a Read Holding Registers message which enquires
the values of word parameters 121 and 122 (see Table 4-2).
MODBUS Function 17 (Report Slave ID) is not supported.
9.2.1Message Formats
The first character of every message is the Controller address, in the range 1 - 255 and 0 for
broadcast messages.
The second character is always the Function Number.
The contents of the remainder of the message depends upon this Function Number.
In most cases the Controller is required to reply to the message by echoing the address and
Function Number.
Broadcast messages are supported at address 0 (to which the CD3000M responds by
taking some action without sending back any reply).
Data is transmitted as eight-bit binary bytes with one start bit, one stop bit and parity checking set
to none. A message is terminated simply by a delay of more than three character lengths at the
Baud rate used(in this case 9600 baud); any character received after such a delay is considered to
be the potential address at the start of a new message.
CD3000M-2PH from 125A to 700A
31
Function Number
03
16
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