Structure Of A Modbus Telegram - JUMO LOGOSCREEN nt Interface Description

4.4 Structure of a Modbus telegram

Data structure
A minimum response time can be set in the controller, in the menu
item Configuration
time that must elapse before an answer is transmitted (0 — 500
msec). If a smaller value is set, then the response time may be
longer than the preset value (because the internal processing time
is longer), the controller will then answer as soon as the internal
processing is completed. A preset time of 0 msec means that the
controller answers with the maximum possible speed.
The minimum response time which can be set is required by the
RS485 interface in the master, in order to switch over the interface
driver from transmit to receive. This parameter is not required for
the RS232 interface.
t This is the waiting time which the master has to observe before
2
initiating a new data request.
for RS232
for RS485
While t and t are running, the master must not present any further data
1 2
requests, since the paperless recorder will either ignore them or declare them
to be invalid.
All telegrams have the same structure:
Slave address
1 byte
Each telegram contains four fields:
Slave address the device address of a specific paperless recorder
Function code function selection (read/write a word)
Data field contains the information (depending on the function code):
- word address / bit address
- word number / bit number
- word value /bit values
Checksum detection of transmission errors
4 Modbus protocol description
Interface. This setting is the minimum waiting
at least 3.5 x the transmission time for
one character (this time depends on the
baud rate)
60msec
Function code
1 byte
Data field Checksum
x byte(s)
CRC16
2 bytes
17