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The Novatech 1732 transmitter has the ability to work as a Modbus slave node on serial RS-232 or 3-wire
RS-485 via RTU mode transmissions. By accessing information stored in the input and holding registers it is
possible to access runtime variables, alarm conditions and modify the device configuration.
The implementation of the Modbus protocol is limited to the specific command set relevant to reading and
writing register variables. Attempting to send unrecognised commands will result in appropriate error
responses
MODBUS™ Functions Supported:-
0x03:
Read Holding Registers
0x04:
Read Input Registers
0x06:
Write Single Holding Register
(return query data – for loopback testing)
0x08:
0x10:
Write Multiple Holding Registers
0x16:
Mask Write Holding Register
0x17:
Read/Write Multiple Holding Registers
0x2B:
Encapsulated Interface (read device information)
0x41:
Special Instruction Function
Serial Configuration
The serial configuration for the slave device is accessed in the Commissioning menu of the transmitter.
Default settings are highlighted in bold.
Baud Rate
Parity
Stop Bits
Interface
All holding register addresses contain a single 16 bit value, however some variables span multiple holding
registers to yield a single 32 bit variable.
When reading register values be aware that the byte order of data within registers may differ from that of the
target system.
Modbus Protocol
The Modbus over serial line protocol defines a messaging system for master/slave communications. Only
one master device may be connected to any network with one or several slave nodes.
In unicast mode, the master initiates communication with a specific slave node by sending a request
message. The slave processes the request and returns a reply message containing the requested
information or a confirmation that the request was fulfilled.
Modbus requests are transmitted as a formed request frame with a CRC for data integrity checking. The
frame for each request and reply type will vary somewhat, but will always have the basic structure described
below:
1 byte
1 byte
slave
function
address
code
Detailed information regarding the implementation of the Modbus protocol can be obtained from the website
of the Modbus Organisation.
http://www.modbus-ida.org
Internal Representation of Dates & Alarm Status
The transmitter stores all dates as an unsigned 32bit count of seconds elapsed since 1-Jan 2004.
Alarm status is stored as an integer value:
0
clear
1
self cleared
August 2018
1732 Oxygen Transmitter
2400, 4800, 9600, 19200, 38400, 57600, 115200
Even, Odd, None
1
RS-485, RS-232
up to 255 bytes
data
2
active
3
acknowledged
A
PPENDIX
2 bytes
checksum
4, MODBUS™
Technical Manual
69
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