Basler Electric DECS-250N - Manual (Page 217)

9440500990 Rev D 201
DECS-250N Modbus™ Communication
installation and altered during real-time operation. The DECS-250N Modbus supports baud rates up to
115200. The factory default baud rate is 19200.
DECS-250N systems support RS-485 compatible serial interfaces. This interface is accessible from the
left side panel of the DECS-250N.
Message Framing and Timing Considerations
When receiving a message via the RS-485 communication port, the DECS-250N requires an inter-byte
latency of 3.5 character times before considering the message complete.
Once a valid query is received, the DECS-250N 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 20 provides the response message transmission time (in seconds) and 3.5 character times (in
milliseconds) for various message lengths and baud rates.
Table 20. Timing Considerations
Baud Rate
3.5 Character Time
(ms)
Message Tx Time(s)
128 Bytes 256 Bytes
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
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 167.
Figure 167. Modbus Request/Response Over TCP/IP
A dedicated header is used on TCP/IP to identify the Modbus Application Data Unit. It is called the MBAP
header (Modbus Application Protocol header).
This header provides some differences compared to the Modbus RTU application data unit used on a
serial line:
•
The Modbus ‘slave address’ field usually used on Modbus Serial Line is replaced by a single byte
‘Unit Identifier’ within the MBAP header. The ‘Unit Identifier’ is used to communicate via devices
such as bridges, routers, and gateways that use a single IP address to support multiple
independent Modbus end units.
•
All Modbus requests and responses are designed in such a way that the recipient can verify that
a message is finished. For function codes where the Modbus PDU has a fixed length, the function
code alone is sufficient. For function codes carrying a variable amount of data in the request or
response, the data field includes a byte count.
•
When Modbus is carried over TCP, additional length information is carried in the MBAP header to
allow the recipient to recognize message boundaries even if the message has been split into
multiple packets for transmission. The existence of explicit and implicit length rules and use of a
PDU
Modbus TCP/IP ADU
Function code Data MBAP Header