© Microhard Systems Inc.
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5.0 Network Topologies
Sending this data to the remote units is important for the operation and timing of Fast TDMA
as it uses this information to determine the slot duration, and identifies to each unit when, and
for how long it will be allowed to transmit data. Unlike traditional TDMA, the master unit only
sends synchronization data at the very beginning of the TDMA frame. Other user data can be
sent by the master unit, by assigning it a slot in the TDMA table, but at this point it acts as a
remote unit, rather than a master unit.
Register 212 is used to set the expected packet size of the data received by the master, from
the remote units. This register is only set in the Master unit, and defines the maximum dura-
tion, in bytes, of how long each remote unit is allowed to transmit data. The slot sizes for each
remote unit is identical, and defined by the S212 register.
The slot size for the user data of the Master, is defined by the Master units S112 register, it
does not have to match the slot size of the remote units (S212). As seen in Figure 1, the time
slot for the Master unit (In a PMP network, the Master always has a unit address of 1), is dif-
ferent than the time slots for the other units.
The S112 register on the remote units is the Max Packet Size of the data transmitted to the
master, it has to be equal to or less than the S212 register of the Master, as the master ex-
pects the data to only be this size, or smaller. Essentially if S112 of the remote units is larger
than S212, the data will not fit into the allotted slot, and the data will be lost.
The maximum buffer size is 1580 bytes, which would limit the values of registers S112 and
S212 to 1580 bytes.
The remote units (and the Master) can then use the data above to calculate, when it will be
allowed to transmit data. By knowing which unit in the list, and how long the remote and mas-
ter TDMA slots are, each unit in a Fast TDMA network will know precisely when to transmit its
data.
If a remote misses a Master’s SYNC packet the remote unit will not transmit until a new
SYNC packet is received, if the remote keeps missing the sync packet for a duration longer
than that defined by register S248 - Sync Timeout (in seconds), the remote will go into
search mode and start looking for a master. This duration of search time can be significant as
the master only sends SYNC/TDMA packets at the very start of a TDMA Frame (time de-
pends on size of TDMA table).
Fast TDMA assumes single packets are sent during assigned time slots, if multiple packets of
varying lengths are required, it is best to assign multiple slots to ensure data overhead is tak-
en into consideration. Contact Microhard for more information if required. Retransmissions,
ACK and packet fragmentation are not used in this mode.
