Running the
motor
98
Commander S100 User Guide
8 Communications
8.1 Control Techniques 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 in this section.
8.1.1 MODBUS RTU
Physical layer
* See
Serial Mode
(P4.04)
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 maximum slave response time of 200 ms. The minimum slave response time 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.
The Commander S100 can also a add a transmit delay if the master is not ready to receive data within 1 ms of the drive receiving a message. See
Minimum Serial Comms Transmit Delay
(P4.06).
8.1.2 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)
1 or 2 stop bits*
Parity bits
None, even or odd *
Baud rates
600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 76800, 115200
Message data
SLAVE
ADDRESS
16bit CRC
message data
FUNCTION
CODE
Silent
interval
Master request
Time
frame detect
Slave frame
processing
Slave response
Slave response time
Master request
New master request
can start here
minimum silence
period
minimum silence
period
Summary of Contents for COMMANDER S100
Page 118: ...0478 0650 02...
