905U-G Wireless Gateway
User Manual
Page 19
©September 2004
Chapter 2
OPERATION
2.1
Start-up
The 905G operating software and the database configuration are stored in non-volatile
memory, however the database I/O register values are lost on power failure (in the same way
as a PLC).
On start-up, the 905G sends "start-up poll" messages to remote modules based on the source
address of inputs configured in the database (the start-up messages can be disabled by
configuration). The remote modules respond with update messages for their inputs, which
sets initial values in the 905G I/O database registers. The 905G provides a delay of 5
seconds between each start-up poll, to allow the remote module to respond and to avoid
overloading the radio channel.
If there are a lot of remote modules, then this start-up stage may take a significant time, and
this should be allowed for in the system design. The 905G has an internal battery charger
feature and the use of a back-up battery should be considered if this start-up delay presents a
constraint to system reliability. Start-up polls may be disabled for individual remote modules
in the database configuration.
For the host device, the 905G provides an "Active" signal on the RS232 port (DCD pin 1).
Its purpose is to indicate to the host that the 905G is now processing output messages for the
remote modules. When the 905G powers down (or should an internal fault occur), the
"Active" signal resets (turn “off” or “0”). When the 905G starts-up, it holds the "Active"
signal in a reset condition (“off” or “0”) for a time equal to the number of remote addresses
(or modules) configured times 5 seconds plus any delay if remote addresses are offline. For
example, if there are 20 remote addresses configured in the 905G database, then the “active”
signal will be held in the reset state for 100 seconds (20 x 5). During this period, the 905G
will not change any output values in its database. After this time, the 905G will set the
"Active" signal (to “on” or “1”) - the host can then send messages to the 905G to update the
output values in the database.
2.2
Operation
The 905G database can hold values for 4300 I/O signals plus the 8 on-board I/O. The
database registers (also called I/O registers) can be accessed by both the radio port and the
fieldbus port. The host device can change values in the database via the fieldbus, and the
905G can transmit radio messages out with the new values. Radio messages can be received
with new values for database registers, and these new values can be written to the host device
or read by the host device, via the fieldbus.
The 905G operation must be configured before the 905G will function. Configuration is
achieved by creating a configuration file on a PC and downloading this file to the 905G. The
905G configuration may also be "uploaded" to a PC for viewing and modification. For more
information, refer to the
Configuration
section of this document.
Each I/O register in the 905G database has a 16-bit value. It doesn’t matter if the remote I/O
is digital (discrete), analog or pulse. The host protocol driver in the 905G will convert the 16
bit value into a value that the host will understand. For example, if the host device requests a
