Modbus Rtu; Slave Address; Modbus Registers; Rtu Framing - Emerson unidrive sp User Manual

Parameter Keypad and
Parameter x.00
structure display
7.2.1

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 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
Master request
7.2.2

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.
7.2.3

MODBUS registers

The MODBUS register address range is 16bit (65536 registers) which at
the protocol level is represented by indexes 0 through 65535.
PLC registers
Modicon PLCs typically define 4 register 'files' each containing 65536
registers. Traditionally, the registers are referenced 1 through 65536
rather than 0 through 65535. The register address is therefore
decremented on the master device before passing to the protocol.
404
Parameter Advanced parameter
description format descriptions
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*
300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200
message data
Message data
minimum silence
period
Slave frame
frame detect
processing
Slave response time
Time
www.controltechniques.com
Serial comms
Macros
protocol
Description
16bit CRC
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.
minimum silence
period
Slave response
File type Description
1 Read only bits ("coil")
2 Read / write bits ("coil")
3 Read only 16bit register
4 Read / write 16bit register
The register file type code is NOT transmitted by MODBUS and all
register files can be considered to map onto a single register address
space. However, specific function codes are defined in MODBUS to
support access to the "coil" registers.
All standard CT drive parameters are mapped to register file '4' and the
coil function codes are not required.
CT parameter mapping
All CT products are parameterized using the #menu.param notation.
Indexes 'menu' and 'param' are in the range 0 through 99. The
#menu.param is mapped into the MODBUS register space as menu*100
+ param.
Electronic
Performance
nameplate
Silent
interval
Master request
New master request
can start here
Unidrive SP Advanced User Guide
Issue Number: 10
RFC mode