Honeywell XNX, Техническое руководство

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XNX Universal Transmitter
Appendix B - Modbus Protocol 184
B.1 Modbus and the XNX transmitter
The XNX
®
gas sensor may be fitted 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 configuration
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 first five registers (four data)� The assignment of the first
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 confirm 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.