Structure Of A Modbus Telegram - JUMO mTRON T Manual

NOTE!
A minimum response time can be configured in the central processing unit using the setup
program under CPU > C
minimum waiting time before an answer is transmitted (0 to 500 ms). If a smaller value is set,
then the response time may be longer than the preset value (internal processing takes lon-
ger), the central processing unit responds as soon as internal processing is completed. A pre-
set time of 0 ms means that the device responds with the maximum possible speed.
The minimum response time, which can be set is required by the RS485 interface in the mas-
ter, to switch over the interface drivers from transmit to receive. This parameter is not re-
quired for the RS232 interface.
NOTE!
No data requests from the master are permitted during t
sponse time. Data requests made during t
during the response time will result in the invalidation of all data currently on the bus.
3.4

Structure of a Modbus telegram

Data structure
All telegrams have the same structure:
Slave address
Each telegram contains four fields:
Slave address
Function code
Data field
Checksum
ONFIGURATION LEVEL
Function code
1 byte
Device address of a specific slave
Function selection (reading/writing words)
Contains information (depending on the function code)
- word address/bit address
- number of words/bits
- word value(s)/bit value(s)
Detection of transfer errors
3 Modbus protocol description
> S
ERIAL INTERFACE
and t are ignored by the slave. Data requests
1 2
1 byte
. This preset time is the
and t and during the slave re-
1 2
Data field
x bytes
Checksum
CRC
2 bytes
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