STS DTM.OCS.S, Руководство по эксплуатации, Страница: 14 / 33

Operating manual Digital Transmitter DTM.OCS.S / DTM.OCS.S/N
Communication with the DTM.OCS.S transmitter
14
Doc-Nr. 10.00.0430
5.3.2 General Modbus message frame structure
Message frame:
Modbus message (command or response)
Address FC Code DW1 DW2 DW3
………………………
DWn CRC
1 byte 1 byte 2 bytes 2 bytes 2 bytes 2 bytes 2 bytes
Modbus PDU
Modbus Serial Line PDU
Tab. 3: Modbus message frame
The number of data words can be derived from the definition of the
respective commands. The CRC checksum is divided into two
individual bytes with the low byte being transmitted first, followed
by the high byte. The CRC checksum is calculated using address,
function code and the data words. If the message is faulty (e.g. an
invalid CRC checksum is provided), no response is given.
5.4 Modbus STS commands
5.4.1 STS command structure
For the communication with the DTM.OCS.S, a custom STS
function code is used: 0x64 (100
10
).
FC Code data size STS command and its parameter(s) (ASCII encoded strings)
0x64 N 1
≤
N
≤
250 bytes
Modbus PDU
Tab. 4: STS command structure
The STS command and its parameter(s) are encoded as readable
ASCII text strings.
As they are embedded in the PDU’s payload field, the data frame
remains Modbus RTU compatible.
5.4.2 Sensor response
As required to be compliant with the Modbus standard, a sensor
response is carried out only upon a request from the master
device. The reply is embedded in the PDU’s payload, encoded as
readable ASCII text strings.
Provided a valid Modbus message frame was received, the reply
will contain the requested data, preceded by the received
command string (without parameters). Additionally, the
DTM.OCS.S returns one of the following status messages and
appends it to the requested data: