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Commander SK Advanced User Guide
www.controltechniques.com Issue Number: 9
6 CT Modbus RTU
6.1 CT Modbus RTU specification
This section describes the adaptation of the MODBUS RTU protocol offered on Control Techniques' products. The portable software class which
implements this protocol is also defined.
MODBUS RTU is a master slave system with half-duplex message exchange. The Control Techniques (CT) implementation supports the core
function codes to read and write registers. A scheme to map between MODBUS registers and CT parameters is defined. The CT implementation also
defines a 32bit extension to the standard 16bit register data format.
6.1.1 MODBUS
RTU
Physical layer
RTU framing
The frame has the following basic format
The frame is terminated with a minimum silent period of 3.5 character times (for example, at 19200 baud the minimum silent period is 2ms). Nodes
use the terminating silence period to detect the end of frame and begin frame processing. All frames must therefore be transmitted as a continuous
stream without any gaps greater or equal to the silence period. If an erroneous gap is inserted then receiving nodes may start frame processing early
in which case the CRC will fail and the frame will be discarded.
MODBUS RTU is a master slave system. All master requests, except broadcast requests, will lead to a response from an individual slave. The slave
will respond (i.e. start transmitting the response) within the quoted maximum slave response time (this time is quoted in the data sheet for all Control
Techniques products). The minimum slave response time is also quoted but will never be less than the minimum silent period defined by 3.5 character
times.
If the master request was a broadcast request then the master may transmit a new request once the maximum slave response time has expired.
Attribute
Description
Normal physical layer for multi-drop operation
RS485 2 wire
Bit stream
Standard UART asynchronous symbols with Non Return to Zero (NRZ)
Symbol
Each symbol consists of:-
1 start bit
8 data bits (transmitted least significant bit first)
2 stop bits
Baud rates
2400,4800, 9600, 19200, 38400
Physical layer
UART
RTU framing
Parameter
Database
MODBUS RTU
CMP
X.Y
Message data
SLAVE
ADDRESS
16bit CRC
message data
FUNCTION
CODE
Silent
interval
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