© Microhard Systems Inc.
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5.0 Network Topologies
5.2.2 Adaptive TDMA
Adaptive TDMA allows for the list of slaves to be populated and changed automatically and is
ideal for systems that are changing constantly (i.e mobile applications). In Adaptive TDMA the
user does not create or enter the TDMA list, when the system is first initialized the master will
accept channel requests from slaves and begin to create a TDMA slave list.
As in Standard TDMA mode, the Master modem will sequence through a list of remotes al-
lowing each one, in turn, to transmit its data. If a slave is not in the list, or misses its turn to
send it data it will send a channel request to the Master at the end of the current TDMA cycle,
where the master will accept requests for a specified number of hops. Once the Master radio
hears the request, the unit is added to the list for the next TDMA cycle. On the other hand, if a
slave is assigned a channel and it does not respond it begins to age and unless it begins to
respond it will be removed from the TDMA list.
To configure an Adaptive 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
•
S234 Master Channel Request Timeout (1-254)
•
S235 Routing Time to Live (1-255)
For Adaptive TDMA, set S244=4. (Must be set on the Master and all Slaves).
The Master Channel Request Timeout register S234 determines how many hop intervals the
Master waits for channel requests at the end of each TDMA cycle and allows slaves to submit
channel requests. A large value for S234 adds overhead to the end of each TDMA cycle, so
ideally this should be kept fairly short, however, in very large systems, a too short of a time
will mean making populating the initial TDMA list take many TDMA cycles.
Register S235 defines the aging process of slave units (i.e going out of range, turned off etc)
and removal from the TDMA list. S235 is an index from 1-65535. If set for 65535, the unit will
never be removed from the list. Otherwise the index begins a countdown that is decremented
by the Master at regular intervals when a slave is not responsive. Once data, any data, is re-
ceived by the Master, this index is reset. If the Index reaches 0, the slave unit will be removed
from the TDMA list. If the slave comes back to life (i.e. comes back into range), it can request
a channel at the end of the TDMA frame and be added to the list once again.
The AT&R1 command can be issued on the Master to view the current TDMA table. The
Master unit is always present in the TDMA list as unit address 1.
