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Version RTM1688-2 MODBUS
The MODBUS protocol offers a communication option in addition to the SARAD standard protocol. The
protocol type can – depending on the type of instrument – be selected either by jumper, switch or
menu. The MODBUS protocol implements only a part of the interface functionality and has been
implemented primarily for the cyclic reading of current measuring results. The adjustment of the
configuration parameters as well as the download of time distributions stored in the instrument is not
possible.
Standards
MODBUS APPLICATION PROTOCOL SPECIFICATION V1.1b3
MODBUS over serial line specification and implementation guide V1.02
www.modbus.org
With respect to the standard following communication parameter are defined:
Baud rate: 9600bps or 19200bps (configurable)
Data format: 1 start bit, 8 data bits, 1 parity bit, 1 stop bit (total 11 bit)
Parity: even
Address range: 1...255 (configurable)
Bus timing
Min. period between two frames (t3.5)
Max. period between two bytes within a frame (t1.5)
Response time
Error management
Incomplete frames or frames with invalid check sums will be ignored and result in a client time out.
Invalid or not supported function codes, register addresses and data length settings are responded by
the related exception codes:
Invalid function:
Invalid address:
Invalid number of registers:
Hardware
Instruments with native RS-485 interface (full bus functionality)
Instruments with RS-232 interface with RS-232/RS-485 converter (full bus functionality)
Instruments with RS-232 interface or internal USB/UART converter (point to point connection
without bus functionality – for example host which handles transmission over virtual (COM
port)
Instrument specific implementation of the bus functions
Implemented MODBUS functions
Function code 0x03 (read holding register)
SARAD GmbH 2023
Manual_RTM1688-2_EN_24-02-2023.docx
Code 0x01
Code 0x02
Code 0x03
RTM 1688-2
9600 bps
19200 bps
4.025ms
2.01ms
1.75ms
0.862ms
< 1s
< 1s
13
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