Ct Modbus Rtu Specification - Nidec Unidrive M200 User Manual

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8.6

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 32 bit extension to the standard 16 bit register data format.
8.6.1 MODBUS RTU
Physical layer
Attribute
Normal physical layer for multi-drop operation
Bit stream
Symbol
Baud rates
* The drive will accept a packet with 1 or 2 stop bits but will always transmit 2 stop bits
RTU framing
The frame has the following basic format
SLAVE FUNCTION
ADDRESS CODE
The frame is terminated with a minimum silent period of 3.5 character times (for example, at 19200 baud the minimum silent period is 2 ms). 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 that 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.
The master must implement a message time out to handle transmission errors. This time out period must be set to the maximum slave response time
+ transmission time for the response.
Master request
8.6.2 Slave address
The first byte of the frame is the slave node address. Valid slave node addresses are 1 through 247 decimal. In the master request this byte indicates
the target slave node; in the slave response this byte indicates the address of the slave sending the response.
Global addressing
Address zero addresses all slave nodes on the network. Slave nodes suppress the response messages for broadcast requests.
56
Electrical Getting Basic
installation started parameters
EIA485 2 wire
Standard UART asynchronous symbols with Non Return to Zero (NRZ)
Each symbol consists of:-
1 start bit
8 data bits (transmitted least significant bit first)
2 stop bits*
600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 76800, 115200
message data
Message data
minimum silence
period
Slave frame
frame detect
processing
Slave response time
Time
Running the NV Media
Optimization
motor Card
Description
Slave response
Onboard Advanced
Diagnostics
PLC parameters
16bit CRC
minimum silence
period
Master request
New master request
can start here
Unidrive M200 / M201 Control User Guide
Issue Number: 2
UL Listing
Silent
interval