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microFlow.net Liquid Modbus Communications Manual
Appendix
4 – Appendix
4.1
Modbus Communications Primer
The microFlow.net Liquid Modbus interface is designed to conform to a subset of the
“Modicon Modbus Protocol Reference Guide” PI-MBUS-300 Rev. D (Modicon, Inc.,
Industrial Automation Systems). Modbus can be implemented on various transmission
mediums (such as RS-232 or RS-485 communication ports). Transmission of data is
serial and asynchronous. It is recommended that communications ports 2 or 3 on the
microFlow.net Liquid be used for Modbus communications.
The Host Message:
The host transmits a message on the communications line that
represents a specific query or command. The address specifies which slave device is
to act on the message. The function in the query tells the addressed slave device what
kind of action to perform. The register word specifies what particular internal state/value
of the slave is of interest to the host. The data bytes contain any additional information
that the slave will need to perform the function. For example, function code 03 will query
the slave to read holding registers and respond with their contents. The register field
must contain information telling the slave which register(s) to read and the data field
specifies how many registers to read. The error check or CRC (cyclical redundancy
check) field enables the slave to validate the integrity of the message contents.
The Response:
If the slave makes a normal response, the function byte in the response
is an echo of the function in the query. The data bytes contain the data collected by the
slave, such as register values or status. If an error occurs, the function code is modi-
fied to indicate that the response is an error response, and the data bytes contain a
code that describes the error. The error check field allows the master to confirm that
the message contents are valid.
4.11
RTU Framing
Every Modbus message begins with a silent interval of at least 3.5 character times.
Multiply the character times by the current network baud rate to determine the length of
the silent interval (see T1-T2-T3-T4 in the figure below). Next, the microFlow.net Liquid
address field is transmitted.
Characters for all fields are transmitted as binary bytes. In this manual, characters are
represented by hexadecimal 0-9, A-F. All networked devices constantly monitor the
network bus. This monitoring occurs even during silent intervals. As each microFlow.
net Liquid receives the first field (the address field), it decodes it to determine if it is the
microFlow.net Liquid being addressed.
A second silent interval of at least 3.5 character times follows the last transmitted char-
acter of each message, after which a new message can begin. The new message must
be transmitted as a continuous stream, with no silent interval in excess of 3.5 character
times. If an excessively long silent interval occurs before completion of the frame, the
receiving microFlow.net Liquid will disregard the entire incomplete message and wait
for the address field of the next new message.
If a silent interval is less than 3.5 character times, the receiving microFlow.net Liquid
will be unable to recognize it as the start of a new message and will attempt to read it
as a part of the prior message. These combined messages will result in an invalid value
in the final CRC field, and an error will result. A typical message frame is shown below.
3.5 char.
time delay
ADDRESS
FUNCTION
REGISTER
DATA
CRC
3.5 char.
time delay
1 byte
1 byte
2 bytes
n bytes
2 bytes
The starting 3.5 character time ending delay for one message may be the same actual
delay as the starting 3.5 character time for the next message (there is no need for the
