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Chapter 4: Network Topologies
To view the current TDMA table, issue the AT&R1 command on the Master. The Master unit is always present in the TDMA list as
unit address 1.
4.2.3 GPS Indexed TDMA (ADHOC)
For GPS applications, GPS TDMA may be an ideal operating mode for some customers. Unlike 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, the system survives longer. Data can be broadcast to
all units within range, or to specific units defined by the destination address of each unit.
In GPS TDMA, all units are connected to an external GPS unit that provides a 1 PPS timing signal to 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 it can transmit data. The number of slaves supported is defined by the hop interval (reg-
ister S109) as follows:
Table 4-1. GPS Indexed TDMA Time Slots.
Hop Interval (S109)
# of Slaves # of Time Slots
200 ms
5
5
100 ms
10
10
50 ms
20
20
25 ms
40
40
20 ms
50
50
10 ms
100
100
5 ms
200
200
The valid address range for units is defined by the total number of supported units as seen in the table above. When the hop
interval is configured as 20 ms, 50 units can be supported, and the address ranges 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 1 PPS
from the GPS unit it can begin to send data, and for how long (20 ms).
To configure a GPS Indexed TDMA, the default settings described in 4.2 apply, with the exception that the following registers
(see Section 5.2) on all units must be modified as required:
• S244 Channel Request Mode.
For GPS TDMA, set S244 = 3.
4.2.4 Fast TDMA
A special version of TDMA, “Fast TDMA” has been designed to minimize the TDMA cycle for systems with large numbers of
remote units, while maintaining the opportunity for each unit to be able to transit its data. Fast TDMA is intended to sequence as
many slaves as possible in the shortest amount of time possible. It is based on the following diagram:
Figure 4-5. Fast TDMA operation and timing.
