© Microhard Systems Inc.
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5.0 Network Topologies
5.2.3 GPS Indexed TDMA (ADHOC)
For GPS applications, GPS TDMA may be an ideal operating mode for some customers. Un-
like other types of TDMA, GPS TDMA doesn’t use master units to synchronize and maintain
timing of the network. In a traditional system if there was a problem with the Master the entire
system would be inoperable, so by not using a master, system survivability is increased. Data
can be broadcast to all units within range, or to specific units defined by the destination ad-
dress of each unit.
In GPS TDMA all units are connected to an external GPS unit that provides a 1PPS timing
signal to radio (Contact Microhard Systems for specifications of GPS signal and input into
radio). The entire TDMA frame is then exactly 1 second. The 1 second frame is then divided
into time slots and units are addressed in such a way that the unit address equals the time
slot in which they can transmit data. The number of slaves supported is defined by the hop
interval (register S109) as follows:
The valid address range for units is defined by the total number of supported unit as seen in
the table above. In the case where the hop interval is configured as 20ms, 50 units can be
supported, the address range is from 1 to 50. The unit address determines which time slot the
unit is assigned. For example, unit 7, is assigned time slot 7. The unit can then determine
when after the 1PPS from the GPS unit it can begin to send data, and for how long (20ms).
To configure an GPS Indexed TDMA, the default settings described in 5.2 are applicable, with
the exception that the following registers (ref. Section 6.2)
on all units must be modified as
required:
•
S244 Channel Request Mode
For GPS TDMA, set S244=3.
Hop Interval (S109)
# of Slaves
# of Time Slots
200ms
5
5
100ms
10
10
50ms
20
20
25ms
40
40
20ms
50
50
10ms
100
100
5ms
200
200
Table 5-1: GPS Indexed TDMA Time Slots
