Honeywell Midas, User Manual

Page: 148 / 160

Midas
®
Gas Detector
A-6
A.2 Sending Commands to the Midas
®
The Midas
®
can accept eleven commands over
Modbus as listed in Table 3. It is possible to reset
alarms, flip relays and perform similar tasks over the
network. All commands are conveyed by writing 4
bytes to special registers as listed in table 2. This may
be accomplished with command 06 (“write special
register”) and command 16 (“write multiple registers”).
For example, resetting alarms is accomplished by
writing 350 to holding register 40021 and writing 13862
to holding register 40022.
The Midas
®
Modbus implementation avoids the
standard “write coil” command in order to increase
resistance to malicious communication over the
Internet. Furthermore, Honeywell Analytics strongly
recommends that any Midas
®
connected to the
Internet be protected from unauthorized commands
by a firewall.
The Modbus commands to manipulate the relays
have no effect unless the relays are appropriately
configured. This can be accomplished via the front
“SEt/FLt” menu or by the Fault Configuration web
page. By default, remote control is disabled.
Table A2. User-Writable Modbus Holding Registers
Ref. Addr.
Reg. Name
Function
40021 CMD1 user command register 1
40022 CMD2 user command register 2
40013 A1T1
Alarm 1 threshold in floating point format,
scaling same as FCN1 and FCN2
40014 A1T2
40015 A2T1
Alarm 2 threshold in floating point format,
scaling same as FCN1 and FCN2
40016A2T2
Table A3. Effect of Writing to Modbus Holding Registers
CMD1
40021
CMD2
40022
Action
0x015E 0x3626 reset alarms and faults
0x025E 0x3626 Inhibit Alarms
0x035E 0x3626 Inhibit Alarms and Faults
0x045E 0x3626 Inhibit All
0x055E 0x3626 No Inhibit
0x065E 0x3626 Energize relay1
0x075E 0x3626 De-energize relay1
0x085E 0x3626 Energize relay2
0x095E 0x3626 De-energize relay2
0x0A5E 0x3626 Energize relay3
0x0B5E 0x3626 De-energize relay3
any other command ignored