Modbus Slave Interface; Implementation - Bin Master C-100MB Operator's Manual

5.0 Modbus Slave Interface

The section specifies how the Model C-100MB SmartBob console is implemented as a Modbus slave at
the EX-485 port and how to use Modbus to interface to it.
For Modbus specifications, refer to the following documents online at www.modbus.org:
Modbus Application Protocol Specification, v1.1b
•
Modbus over Serial Line Specification and Implementation Guide, v1.02
•

5.1 Implementation

Communications flow is per the Modbus Serial Line specification.
implementation on the C-100MB:
The following character formats are supported:
•
RTU mode
o
ASCII mode
o
The master should not send another request until a Modbus frame is detected or a response
•
timeout expires.
If a response timeout expires, the request may be transmitted again and as many times as
•
necessary for the controlling application. Generally, three attempts should be sufficient.
The length of a response timeout should vary with the controlling application based on the data
•
rate and the maximum frame size expected. Following is a guideline:
3.0 sec minimum at 2400 bps
o
1.5 sec minimum at 4800 bps
o
1.0 sec minimum at 9600 bps and faster data rates
o
The Modbus slave will respond with an exception code if any of the following are requested:
•
An unsupported or illegal function code
o
An unsupported or illegal data value
o
An unsupported or illegal address value
o
Broadcast mode is not supported. All requests must be addressed to a C-100MB and a response
•
is to be expected.
As per the Modbus specification, only one master is supported on the network.
•
The registers should be formatted and sent zero-based. This means that holding register 4x0001
•
would be sent as 0 and holding register 4x0100 would be sent as 99.
925-0301 Rev C
8 data bits, Even parity, 1 stop bit (default)
8 data bits, Odd parity, 1 stop bit
8 data bits, No parity, 2 stop bits
7 data bits, Even parity, 1 stop bit
7 data bits, Odd parity, 1 stop bit
7 data bits, No parity, 2 stop bits
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Following are specifics to the