NS 5.14 Firmware
3–12 4900002262 rev. B 3-23-21
Modbus Communications Protocol
Modbus is a serial communications protocol published by Modicon in 1979 for
use with its programmable logic controllers (PLCs). It has become a de facto
standard communications protocol in industry, and is now the most commonly
available means of connecting industrial electronic devices. Modbus is used
extensively in lieu of other communications protocols because it is openly
published and royalty-free, relatively easy to deploy, and capable of moving
raw bits or words without placing many restrictions on vendors.
Modbus enables communication between many devices connected to the same
network, for example, a supervisory computer with a remote terminal unit
(RTU) in supervisory control and data acquisition (SCADA) systems.
The SpectraSensors analyzer acts as a server in a client/server(s) network of
devices. It can receive queries from a client and send responses back using
either Gould Modbus RTU protocol or Daniel Extended Modbus RTU protocol.
Framing/Protocol
The transmission mode used to communicate is either Gould Modbus RTU or
Daniel Modbus RTU with port parameters 9600 to 115200 (baud rate), 8 (data
bits), 1 (stop bit), no (parity), and none (flow control/handshake).
The transmission mode is set by the user via the Modbus Mode parameter
(see “To change parameters in Mode 2” on page 2-15).
Functions
Available functions are 0x03 (read holding registers), 0x06 (write to a single
register), 0x10 (write to multiple registers), and 0x2B (read device
identification).
Reading/Writing in Daniel Modbus mode
Daniel Modbus supports three types of registers: short integer, long integer and
floating point. Each “short integer” register is two bytes in length and will
contain an integer value. Each “long integer” register is four bytes in length and
will contain an integer value and each “floating point” register is four bytes in
length and will contain a floating point value.
Reading/Writing in Gould Modbus mode
Gould Modbus supports three types of variable data, short integer, long integer
and floating point, but all registers are addressed as word (two byte) registers.
The generic serial output (HyperTerminal) is disabled if either
Gould or Daniel Modbus is selected.
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