Modbus; Programming The Host; Addressing Conventions And Register Values; Modbus Cables - Honeywell SEARCHLINE EXCEL PLUS Technical Manual

10.5 Modbus
The Searchline Excel Plus & Searchline Excel Edge detector provides a facility for Modbus digital communication between
Searchline Excel Plus & Searchline Excel Edge and an external controller. Modbus is a widely-supported digital data
communication protocol that provides a standardised set of data communication protocols. The principal features of
the Modbus Interface are:
• Operates as a Modbus Remote Terminal Unit (RTU) responder device node.
• Supports functions 03 and 06 of the Modbus protocol.
• Provides gas concentration reading and alarm status for Inhibit, Fault, Alarm and Warning.
• RS-485 half-duplex communication with multi-drop capability.
• Asynchronous serial link configurable for baud rate and parity.
NOTE
The Modbus broadcast commands are not supported and will be ignored.
10.5.1 Programming the Host
The host computer system must be programmed to interpret the signal and status data received from Searchline Excel Plus
& Searchline Excel Edge as there is no standard Modbus format for the communication of data from gas leak detection
systems.
It is recommended that the host computer system should, as a minimum, be programmed to use Function 03 to collect
alarm and status data from Searchline Excel Plus & Searchline Excel Edge.
Full details of the Modbus protocol can be found in the Modicon Modbus Protocol Reference Guide PI-MBUS-300 available
at www.modbus.org.
CAUTION
!
Modbus does not work if TDA and TDB (A in and B in) pins are transposed.
10.5.2 Addressing Conventions and Register Values
The addressing conventions and register values used follow those set by Modicon.
4xxxx – The Holding registers are used to read Searchline Excel Plus & Searchline Excel Edge alarm status and
few configuration parameters. An address range is used to query the event record and store the result.
The write-only registers can be written singly using Function code 06.
Modbus communication settings can be updated using the write registers.
10.5.3 Modbus Cables
The cable should be an RS-485 shielded data communication cable suited to the installation distance, ambient and
environmental factors.
Note:
• Spurs shall not be longer than 1m (39 in.) each, and shall not exceed 10m (33 ft.) combined total.
• The maximum cable length without line repeaters is 1.2km (3900ft).
• The minimum wire size is 1.5 mm
10.5.4 Modbus Electrical Connections
Modbus connections are provided through the terminals in the wiring compartment of the Searchline Excel Plus &
Searchline Excel Edge detector. Details of the wiring connections can be found in Chapter 6.2.1 of this manual.
10.5.5 Modbus Termination Resistor
If Searchline Excel Plus / Edge is the last node in a Modbus system highway a 120 Ω termination resistor between A and B
must be fitted to avoid reflections on the RS-485 circuit.
Note
The 120 Ω termination resistor is NOT supplied.
(15 AWG), maximum is 2.5 mm
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Searchline Excel™ Plus / Searchline Excel™ Edge – Technical Manual
Chapter 10 Communications
(13 AWG).
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