Emerson DL8000 - Modbus Configuration General Tab

DL8000 Preset Instruction Manual
Revised February-2016 Modbus Communications B-3
B.2.1 Modbus Configuration General Tab
Complete the following steps to configure the Modbus Configuration
screen’s General tab.
1. Review the values in the following fields:
Field
Description
Comm Port
Indicates the port to configure. Click  to display
additional available ports.
Byte Order
Selects the order of data bytes in a transmission. You
can also reverse requests using this selection. Least
Significant Byte First is the default value.
Note: This selection affects only the Modbus
message’s data field. It has no effect on the
data for Function Codes 01, 02, and 05.
Comm Mode
Indicates the communication mode. Select either
RTU or ASCII.
Note: In either mode, the transmitting device places
the Modbus message into a frame with a
known beginning and ending point. See
Tables A-1 and A-2.
RTU
Remote Terminal Unit (RTU) mode allows
for greater character density and better
data throughput than ASCII for the same
baud rate. Each message is transmitted in
a continuous stream. Data is sent in 8-bit
binary characters. RTU mode uses Cyclic
Redundancy Check (CRC) error checking.
RTU is the default value.
ASCII
American Standard Code for Information
Interchange (ASCII) mode represents each
8-bit byte of data as two ASCII characters
that are the hexadecimal representation of
the value. This allows the messages to be
read with the use of a dumb terminal, but
uses twice as many characters as the RTU
mode. Each character sent is composed of
a Start bit, 7 or 8 Data bits, and one or two
Stop bits with Even, Odd, or No parity.
ASCII mode uses Longitudinal Redundancy
Checking (LRC) error checking.
Table B-1. RTU Message Framing
Begin of Frame
Address
Function
Data
CRC Error Check
End
T1-T2-T3-T4
1 Byte
1 Byte
N x 1 Byte
2 Bytes
T1-T2-T3-T4
Table B-2. ASCII Message Framing
Begin of Frame
Address
Function
Data
LRC Error Check
End
:
2 Characters
2 Characters
N Characters
2 Characters
CRLF