Nidec Unidrive M200 - CT Modbus RTU Specification

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56 Unidrive M200 / M201 Control User Guide

Issue Number: 2

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

* 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

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.

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.

Attribute Description

Normal physical layer for multi-drop operation EIA485 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 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 76800, 115200