© Microhard Systems Inc.
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5.0 Network Topologies
5.2.1 Point-to-Multipoint TDMA (Standard TDMA)
Time Division Multiple Access (TDMA) is available as a special form of the PMP network to-
pology.
In Standard TDMA mode, a list of remote units is configured in the Master modem, the Mas-
ter unit then cycles through the list and indicates to the remote when it is able to transmit its
data. The remote unit would then begin sending data, if it had data to send, and then release
the channel when no longer needed. This would indicate to the master unit to queue the next
unit and so on.
In this mode each slave unit has the channel or right to broadcast, for varying lengths of time,
and if a remote did not respond, the Master would need to time out before moving on to the
next unit in the list. The maximum number of Remotes which can communicate with a Master
in this configuration is 128.
To configure a Standard TDMA network, the default settings described in 5.2 are applicable,
with the exception that the following registers (ref. Section 6.2)
on the Master must be modi-
fied as required:
•
S244 Channel Request Mode
•
S251 Master Hop Allocation Timeout
For TDMA, set S244=1. (Must be set on Master and all Slaves)
The default for S251 is 10 (hop intervals). If the system is to be deployed in a ‘clean’ RF envi-
ronment, this number should perhaps be reduced to 2 or 3 to provide enough time for the
Slave to initiate its response but to not potentially waste a significant number of hop intervals
waiting for an unresponsive Slave to send data.
In addition, the following AT commands (ref. Section 6.1) are used to populate, view and
change the Registered Slaves List:
•
T?
view entire Registered Slaves List
•
T
n
=
UA
enter a Slave’s Unit Address (UA) into the Registered
Slave’s List item number
n,
where
n
=0-127, and
UA
= 0-
65534 (selecting a UA value of 0 terminates the list)
•
T
n
?
view Registered Slaves List entry number
n,
where
n=
0-
127. Response is UA of List entry
The default Registered Slaves list consists of 128 entries (0-127), populated with Unit Ad-
dresses of 2 thru 129 respectively.
On the following page is an example to illustrate basic TDMA operation. For an actual de-
ployment, application-specific parameters must be considered and other various modem con-
figuration options optimized accordingly.
