Manual – Starflow QSD Model 6527B and 6537A
Unidata Manual - 6527B and 6537A Starflow QSD User Manual 08 05 2023.docx
Page 27
5.2. Modbus RS485 Operation
The Starflow QSD can be operated as a Modbus RTU Slave sensor. The Baud rate and
parity selected must be configured prior to using the instrument. The instrument supports
configurable Baud rates for the RS-485 Communication Port (the SDI-12 Port is fixed to
1200/9600 as per the SDI-12 standard). There are two configuration parameters (baud rate
& parity), and these may be changed via Modbus commands.
When changing these, the new settings come into effect AFTER the response has been sent
to the Modbus PC utility. To continue communicating via RS-485, the PC utility’s baud rate
settings MUST then be changed to match that of the Starflow QSD.
RS485 Baud rate and Parity setting options can be configured using Modbus commands
(refer to QSD Modbus supplement document).
5.2.1. Reading the measured parameters via Modbus
Physical links requirement: RS485 - low power, 2V, three-wire (A/B signals & ground),
differential, half-duplex; at 9600bps (support up to 1000m cable, at 19200bps up to 500m
cable, at 115Kkps up to 100m cable) and it supports PP, MD & MP protocols.
Protocol requirement:
MODBUS (Modicon) - variable speed (9600 & 19200), MD over
RS485. For PC configuration, a USB to RS485 converter is required. The instrument must
be powered via 12V and ground; A- and B+ signals will need to be connected as per the
wiring table in specifications
section 2.0
.
The instrument supports the
Modbus RTU communication protocol
; this is the format of
choice for serial communications, such as RS485/RS232, as it is the most efficient.
The general message structure will consist of the device address, function code, data
payload and checksum.
5.2.2. Master Format
Device Address
Function Code
Data Payload
CRC
Device address – 1 byte field ranging from 1 to 247. Broadcast address 0
Function code – 1 byte field with a value range 1-127 representing the standard or
extended function code
Data payload – 0-N bytes with response data from the device. Error response will be a
1 byte value 1-255.
CRC – 2 bytes computed mathematically.
