User's Manual

MDS 05-2708A01, Rev. D MDS TransNET I&O Guide 55
All Remotes synchronize to a corresponding Master. This can be the “real
Master” (the MODE M unit), or it can be a repeater “Extension” that derives
synchronization from the “real Master.”
Payload polls/packets broadcast from the network Master will be repeated to
all levels of the network, either directly to Remotes, or through network
repeaters—the Extension station. The targeted Remote responds to the poll
following the same path back to the Master.
Synchronization Messages
Remotes acquire synchronization and configuration information via
SYNC
messages. They can synchronize to the Master (the MODE M unit) or to any
valid Extension (a MODE X unit).
The Master will always transmit SYNC messages. An Extension will only
start sending SYNC messages after synchronization is achieved with its
Master.
The ability to synchronize to a given radio is further qualified by the sender’s
Extended Address (XADDR) and by the receiver’s Synchronization Qualifiers
(XMAP, XPRI, and XRSSI).
When a primary is specified (XPRI is 0...31), a radio will always attempt to
find the primary first. If 30 seconds elapses and the primary is not found, then
the radio attempts to synchronize with any non-primary radio in the
XMAP
list.
Once every 30 minutes, if a primary is defined, the radio will check its
synchronization source. If the radio is synchronized to a unit other than the
primary, then the current
RSSI value is compared to the XRSSI value. If RSSI
is less than XRSSI (or if XRSSI is NONE) the radio will force a
loss-of-synchronization, and hunt for the primary again (as described in the
previous paragraph).
By default, Extensions (and the Master) begin with
XADDR 0. Synchroniza-
tion qualifiers are set to XMAP 0, XPRI 0, and XRSSI NONE, respectively. This
default configuration allows any radio to hear the Master. When an Extension
is added, the extended address of the Extension radio must be set to a unique
value. All Remotes that need to hear that extension can specify this either by
designating the extension as the primary (XPRI), or by including it in their list
of valid synchronization sources (XMAP).
9.3 Using AT Commands
A TransNET network may be configured to support protocols employing
Hayes-Compatible modem commands through the radio’s AT Mode. In this
mode, TransNET units can provide a communications replacement for
dial-up modems where the RTUs and the protocol do not contain address-
ability, and the establishment of a direct-communications link is the only way
to determine if the RTU has data ready to be sent. This requirement is
common in many older SCADA systems which were developed for direct