B&B Electronics MES1A - Manual (Page 58)

Modbus Basics
Manual Documentation Number: MES1A/MES1B-2106m PN7138-rev001
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it sends a message containing a device address, followed by a function
code, any data that maybe required, and an error check field. The addressed
slave responds with a similar message structure. Typically it repeats back its
address and the function code, and then sends a field indicating the number
of bytes of data it is sending, followed by the data and the error check field.
Slave addresses can range from 1 to 247. Function codes include several
common ones typically used in all applications, and additional ones that may
be implemented in specific cases. Common function codes include: Read
Coil Status (01), Read Input Status (02), Read Holding Registers (03) and
Read Input Registers (04).
When a master sends a message to a slave it expects to receive a valid
response within certain length of time. If the slave does not receive the
message, or if the slave receives the message but an error is detected, it does
not respond. If the slave cannot respond appropriately for some other reason
(e.g. it does not recognize the function code), it will return a message
containing an exception response.
Modbus/TCP
Modbus/TCP is a much more recent development, created to allow Modbus
ASCII/RTU protocol to be carried over TCP/IP-based networks.
Modbus/TCP embeds Modbus messages inside TCP/IP frames. Although the
implementation is fairly simple, characteristics associated with networking
add some challenges. For example, because Modbus masters expect and
require responses to their polls within a certain time frame, the non-
deterministic (and other) aspects of TCP/IP networks have to be considered.
Modbus/TCP sets up connections between nodes on the network, sending
requests via TCP in a half-duplex fashion. TCP allows multiple requests to
be ‘pipelined’, or queued in a buffer waiting to be serviced. Modbus/TCP has
the capability to use transaction identifiers (sequence numbers), but some
early manufacturers of Modbus/TCP equipment did not use them. In some
cases this can create problems for Modbus/RTU devices connected to the
network through interfaces. If a request from a master is lost, or contains an
improper function code, the slave does not respond. The master may re-
initiate the request, or there may be other requests in the pipeline. Responses
to later, or other, requests may become out of sync and the master may match
the wrong response with a request. The resulting symptoms could range from
appearing to receive wrong data from a slave, to slaves going off-line, to a
gradually slowing down of response time.