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XNX Universal Transmitter
Appendix B - Modbus Protocol
183
B.1 Modbus and the XNX transmitter
The XNX
®
gas sensor may be tted with the optional Modbus
®
interface card (P/N XNX-MB). Authoritative information on
the Modbus protocol can be found at www.modbus.org.
The XNX supports Modbus/RTU over an RS-485 physical
layer. The interface is isolated and includes a switchable 120
Ohm termination resistor. Baud rates from 1200 to 38,400 are
supported with 19,200 as the default (8 data bits, even parity, 1
stop bit).
Most of the operations that are possible with the HART
®
and
local user interfaces can also be performed using the Modbus
interface. This includes test, calibration and conguration
operations. This appendix describes only how to monitor XNX
status using Modbus.
The zero calibration procedure should be performed prior to
the span calibration. The calibration procedure is in Section
3.2.1.
Some of the relevant Modbus holding registers are listed in the
following table. In most installations, the XNX transmitter reads
only the rst ve registers (four data). The assignment of the rst
eight registers (or six data) is identical to the Honeywell Analytics
XCD gas sensor.
Building an effective Modbus automatic gas detection system
requires checking for faults (using iFaultWarnNumber or
iAlmFltLev) and checking iMonitoringState to conrm that the
XNX is not inhibited or in calibration. The pseudo code example
in Figure 247 suggests computation that should be made in
external automation equipment.
See Section 2.3.4 for information on installing the optional
Modbus hardware. See Section 2.5.1 for information on
setting the Modbus baud rate and address using the local
user interface. See Section A.1.1 for information on setting the
Modbus parameters using the HART interface.
if(
((fCurrentConc < TLV) or (iAlmFltLev & 3 == 0))
// low concentration
and
((iFaultWarnNumber < 1000) or (iAlmFltLev & 64 == 0))
// no fault
and
((iMonitoringState == 1) or (iMonitoringState == 7))
// not inhibited
and
(
(Transport layer SW indicates good Comm.)
or
(iHeartBeat changes every 5 seconds)
) // Modbus link healthy
) Then the area is safe.
Figure 247. Modbus Pseudo Code Example
Modbus connections are shown in the flollowing figure.
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