Specifications

Halogen Gas Monitor – MultiZone Remote Display
Instruction 3015-5157 51
and registers are used to communicate with the HGM-MZ directly or through the HGM-RD. If the
communications is through the HGM-RD, it monitors each MODBUS message to determine if the message is
intended for one of the HGM-MZ’s it is connected to. If it is, the HGM-RD passes the message through to the
HGM-MZ’s. If it is not, the message is not passed through. The HGM-RD does not make any modifications
to MODBUS messages. It simply passes the query through to the HGM-MZ, and passes the response back
to the MODBUS master. In other words, it allows the HGM-MZ’s to be logically connected to the MODBUS
network, when physically they are connected to the local HGM-RD network. It is very important
to understand
that the HGM-RD will only pass messages through to the HGM-MZ when the HGM-RD is either in the
“SYSTEM” screen or the “ZONE VIEW” screen. If the HGM-RD is in any other screen
, it will return a
MODBUS “busy” exception response (exception code 06).
All HGM-RD screens, except the SYSTEM and ZONE VIEW screens, have a 10 minute timeout interval, after
which the screen will return to either the SYSTEM or ZONE VIEW screen, depending on which one it was last
in. Also, the HGM-RD can be password protected such that a password entry is required in order to view
screens other than the SYSTEM or ZONE VIEW screen.
Key Comm Protocol Parameters
MODBUS Mode: RTU only
HGM-MZ Baud Rate: Default is 19,200. Programmable as defined in System data register
Parity: No Parity
Stop Bits: Default is 1. Can be set for 2 via System data register
Maximum Response Time: 4000mS when directly accessing the HGM-MZ. 8,000mS when accessing the
HGM-MZ through the HGM-RD.
Error Checking: CRC per MODBUS specifications
Also of note is the fact that all data
sent out from the HGM-400 is in “little endian” byte order (Least significant
byte followed by most significant byte). This should be taken into account if the master that process the data
is a “big endian” type. Non-data information (starting address, number of points, etc.) follows normal
MODBUS protocol, which is Big Endian.