Winco DGC-2020 - Manual (Page 160)

B-2 DGC-2020 Modbus
™ Communication
9400200990 Rev B
Data Block Field
The query Data block contains additional information needed by the slave to perform the requested
function. The response Data block contains data collected by the slave for the queried function. An error
response will substitute an Exception Response Code for the Data Block. The length of this field varies
with each query.
Error Check Field
The Error Check field provides a method for the slave to validate the integrity of the query message
contents and allows the master to confirm the validity of response message contents. This field is 2 bytes.
Serial Transmission Details
A standard Modbus network offers 2 transmission modes for communication: ASCII or RTU. The DGC-
2020 supports only the RTU (Remote Terminal Unit) mode.
Each 8-bit byte in a message contains two 4-bit hexadecimal characters. The message is transmitted in a
continuous stream with the LSB of each byte of data transmitted first. Transmission of each 8-bit data
byte occurs with 1 start bit and 1 stop bit. A ninth data bit is added when parity is selected. Parity
checking is user-configurable to even, odd or none. The transmission baud rate is also user-configurable,
and both parity and baud rate can be altered during real-time operation. If altered, the new baud rate and
/ or parity will not be enforced until the response message to the current query has completed. The DGC-
2020 supported baud rates are 9600, 4800, 2400, and 1200.
Message Framing / Timing Considerations
When receiving a message, the DGC-2020 will allow maximum inter-byte latency up to 3.5 - 4.0 character
times before considering the message complete.
Once a valid query is received, the DGC-2020 waits 10 msec. before responding.
Error Handling and Exception Responses
Any query received that contains a non-existent device address, a framing error or CRC error will be
ignored - no response will be transmitted. Queries addressed to a DGC-2020 with an unsupported
function code, unsupported register references, or illegal values in the data block will result in an error
response message with an Exception Response Code. The Exception Response codes supported by the
DGC-2020 are listed in Table B-1.
Table B-1. Exception Response Codes
Code Name Meaning
01 Illegal Function
The query Function/Subfunction Code is unsupported; query read of
more than 125 registers; query preset of more than 100 registers
02 Illegal Data Address
A register referenced in the data block does not support queried
read/write; query preset of a subset of a numerical register group.
03 Illegal Data Value
A preset register data block contains an incorrect number of bytes or
one or more data values out of range.
DETAILED DGC-2020 MESSAGE DEFINITION
Device Address
The DGC-2020 Device Address can be any value in the Modbus protocol Device Address range (1 - 247).
A query with a Device Address of 0 signifies a Broadcast message to all slaves - the connected DGC-
2020s will not respond to the broadcast query.
Function Code and Data Block
The DGC-2020 maps all parameters into the Modicon 984 Holding Register address space (4XXXX) and
supports the following Function Codes:
• Function 03 - Read Holding Registers
• Function 6 - Preset Single Register, Non-Broadcast & Broadcast