ABB M101 Manual page 17

I NTR O D U CT IO N
2.2.1.1. The Query
The function code (FC) in the query tells the addressed slave device what kind of action to
perform. 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 data field must contain the information telling
the slave which register to start at and how many registers to read. The error check field
provides a method for the slave to validate the integrity of the message contents.
2.2.1.2. The Response
If the slave makes a normal response, the function code in the response is an echo of the
function code 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 modified 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.
2.2.1.3. Characteristics
Certain characteristics of the MODBUS protocol, as specified by the reference document, are
fixed such as the frame format, frame sequences, handling of communication errors and
exception conditions, and the functions performed. In case of the MNS Digital Gateway, the
transmission mode is also limited to RTU or TCP.
Other characteristics are user selectable. These include a choice of transmission medium,
baud rate and character parity, number of stop bits. These parameters cannot be changed
while the communication interface is active.
The OSI layers 1, 2, and 7 are implemented in the MNS Digital Gateway
Layer 1, 2:
In these layers the physical sending and receiving of bytes, i.e. triggering of the interface
hardware including monitoring of timeouts and CRC-Check generation as well as processing
of addresses is realized. Upon receipt, the fault states, time-out and CRC-Error are being
detected and treated according to the MODBUS RTU specification.
Layer 7:
In this layer the analysis and treatment of the function codes (FC) is implemented. This
includes processing the received commands (read and write of MODBUS-registers) and
generation of the response-message together with the addressed data.
2.2.1.4. Mode of Transmission
The mode of transmission is the structure of the individual units of information within a
message, and the numbering system used to transmit the data. Two modes of transmission
are available for use in a standard MODBUS communication, ASCII (American Standard Code
for Information Interchange), and RTU (Remote Terminal Unit). Both modes provide the same
capabilities for communication. Selecting ASCII or RTU mode defines the bit contents of
message fields, and how information is packed and decoded.
MNS Digital Gateway does not support ASCII transmission.
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