9440300990 28-3
DECS-250 Modbus
®
Communication
Modbus
®
Over Serial Line
Message Structure
Master initiated queries and DECS-250 responses share the same message structure. Each message is
comprised of four message fields. They are:
• Device Address (1 byte)
• Function Code (1 byte)
• Data Block (n bytes)
• Error Check field (2 bytes)
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 one start bit and either one or two stop bits. Parity checking is performed, when enabled,
and can be either odd or even. The transmission baud rate is user-selectable, and can be set at
installation and altered during real-time operation. The DECS-250 Modbus supports baud rates up to
115200. The factory default baud rate is 19200.
DECS-250 systems support RS-485 compatible serial interfaces. This interface is accessible from the left
side panel of the DECS-250.
Message Framing and Timing Considerations
When receiving a message via the RS-485 communication port, the DECS-250 requires an inter-byte
latency of 3.5 character times before considering the message complete.
Once a valid query is received, the DECS-250 waits a specified amount of time before responding. This
time delay is set on the Modbus Setup screen under Communications in BESTCOMSPlus
®
. This
parameter contains a value from 10 - 10,000 milliseconds. The default value is 10 milliseconds.
Table 28-1 provides the response message transmission time (in seconds) and 3.5 character times (in
milliseconds) for various message lengths and baud rates.
Table 28-1. Timing Considerations
Baud Rate
3.5 Character Time
(ms)
Message Tx Time (s)
128 Bytes 256 Bytes
1200 32.08 1.17 2.34
2400 16.04 0.59 1.17
4800 8.021 0.29 0.59
9600 4.0104 0.15 0.29
19200 2.0052 0.07 0.15
38400 1.0026 0.04 0.07
57600 0.6684 0.02 0.04
115200 0.3342 0.01 0.02
Modbus on TCP/IP
Application Data Unit
The following describes the encapsulation of a Modbus request or response when it is carried on a
Modbus TCP/IP network. See Figure 28-2.
Figure 28-2. Modbus Request/Response Over TCP/IP
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