17
4 Modbus protocol description
No data requests from the master are permitted during t
1
and t
2
, because the
paperless recorder will either ignore the request or declare it to be invalid.
4.4 Structure of a Modbus telegram
Data structure All telegrams have the same structure:
Each telegram contains four fields:
Slave address device address of a specific paperless recorder
Function code function selection (read, write words)
Data field Contains information (according to the function code):
• Word address/bit address
• Number of words/bits
•word/bit values
Checksum detection of transmission errors
A minimum response time can be set in the controller under the
menu point Configuration
➔ Interface. This preset time is the
minimum waiting time before an answer is transmitted (0…500
ms). If a smaller value is set, then the response time may be longer
than the preset value (internal processing takes longer), the
controller answers as soon as internal processing is completed.
The preset time of 0 msec means that the controller responds at
the maximum possible speed.
The minimum response time, which can be set is required by the
RS485 interface in the master, to be able to switch over the
interface drivers from transmit to receive. This parameter is not
required for the RS232 interface.
t
2
The master has to observe this waiting time before starting a new
data request.
for RS232 min. 3.5 times the transmission time
required for 1 character (the time
depends on the baud rate)
for RS485 60ms
Slave address Function code Data field Checksum
CRC16
1 byte 1 byte x byte 2 byte
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