Operational Description
In an M1144 system, timing is everything. Data is only transmitted in carefully defined and
synchronised timing "slots". The primary reference for this timing is generated by the Master unit, and
is received (and re-sent) by the slave units to provide a consistent time reference across the whole
network. A slave unit without valid synchronisation cannot transmit.
The diagram (below) shows how the system timings are arranged.
50ms long Slot Burst
Preamble
Framing
Sequence
Checksum
Address
Unit ID
Alarm
Status
1s long Frame
Slot Slot Slot Slot Slot Slot Slot Slot Slot Slot Slot Slot Slot Slot Slot Slot Dead
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Time
800ms long Slots
200ms
8s long Group
Frame 4 Frame 3
Returning Reply Frames
Frame 7
Frame 6
Frame 5
Frame 2
Frame 1
Frame 0
Outgoing Synchronisation Frames
Figure 3: M1144 Synchronised data burst slots of each slave and frame transmission in a group
The basic timing element is a 50ms "slot" (into which a single transmission packet fits, with some
margin for error). 16 slots (and 200ms of inactive dead-time) make an 800ms frame. Each slot in a
frame is assigned to a specific slave unit ID number. A unit can only ever transmit in its assigned slot.
(slot zero is never used)
Eight frames make up an 8 second group, although it is easier to consider the first four
"synchronisation" frames and the second four "reply" frames as almost separate things.
Imagine a system starting from "cold":
In the first frame (zero), the master sends out a synchronising message. This sets the timing "clock" for
all slaves in range (these units we refer to as "zone 1 units". In frame one, all these units re-transmit a
sync message. Units receiving any of these frame 1 messages, but out of range of the master, are the
"zone 2" units
In frame two, the zone two units transmit, and are heard by units further out (out of rnage of both
master and zone 1 slaves), which constitute zone three. Finally, in frame three the zone three slaves
themselves transmit, to the furthest distant units, in zone four.
Zone four units do not transmit a sync message. They wait until frame four and transmit the first
generation of "reply" messages, which are received by the zone three slaves. In frame five, these units
transmit their reply message to zone two, which then transmit to zone 1 in frame six, and finally in
frame seven the zone 1 units transmit to the master.
In this way, you can see that transmitted data radiates outwards (like the ripples in a pond) in frames
0-3, and "bounces back" inwards in frames 4-7.
Reply message bursts carry alarm, and "unit present on system" information from the network back to
the master. On re-sending a reply burst, each slave unit adds its own information to the message
Synchronisation messages carry timing information out through the network, but also contain the sum
of all the network alarm and status information as received by the master in the previous frame. This
is critically important, as the previous-frame network data contained in the synchronising messages
allows every unit (slave, drone or master) to act on any alarm input, and allows every unit to output the
same serial data on its auxiliary port
Radiometrix Ltd
M1144 Application Boards
page 4
RADIOMETRIX [ RADIOMETRIX LTD ]
