DNT500
2008 by RF Monolithics,
Inc.
10
M-0500-0000 Rev D
the RemoteSlotSize are computed automatically by the DNT500 network in this mode.
The user should note that the bandwidth to each remote will change immediately as re-
motes join and leave the network.
TDMA Fixed Slots (Mode 3) is used for applications that have fixed data throughput re-
quirements, such as isochronous voice or streaming telemetry. The slot start time and the
RemoteSlotSize are computed automatically by the DNT500 network in this mode. The
user must set the number of slots.
TDMA with PTT (Mode 4) supports remotes with a "push-to-talk" feature, also referred
to as "listen-mostly" remotes. This mode uses fixed slot allocations. Remotes can be reg-
istered for all but the last slot. The last slot reserved for the group of remotes that are usu-
ally listening, but occasionally need to transmit. In essence, the last slot is a shared chan-
nel for this group of remotes. When one of them has data to send it keys its transmitter
much like a walkie-talkie, hence the name push-to-talk (PTT).
The slot start time and the RemoteSlotSize are computed automatically by the DNT500
network in this mode. The user must specify the number of slots. The last slot is reserved
for the PTT remotes. The user must configure PTT remotes individually to select Mode 4
operation. The network makes no guarantee that PTT remote transmissions will not col-
lide in the shared slot. The user's application must ensure that no more than one PTT re-
mote is using the slot at a time.
2.7.3 Network Configuration Planning
Some planning is necessary for a DNT500 network to coordinate the RF_DataRate,
HopDuration, BaseSlotSize, RemoteSlotSize, MinPacketLength, TxTimeout and
TDMA_MaxNumSlots parameters to achieve a practical configuration. This is true even
for modes that automatically compute some of these parameters. Each parameter has a
limited range of usable values, as shown in the table below:
Parameter
Useable
Range
Value
RF_DataRate 0..3
500,
200,
115.2
and
38.4
kb/s
HopDuration
40..4095
2..204.75
ms
(0.05
ms/count)
TDMA_MaxNumSlots
1..15
max number of TDMA slots (MNS) for remotes
BaseSlotSize
6..232
max number of user data bytes transmitted per hop
RemoteSlotSize
3..243
max number of user data bytes transmitted per hop
MinPacketLength
0..255 0..255
bytes
TxTimeOut
0..255
0..255 ms (1 ms/count)
The highest RF data rate, 500 kb/s, provides the highest throughput and the most flexibil-
ity with respect to the other parameters. The maximum RF power that can be used at
500 kb/s is 19 dBm. The three lower data rates can run up to 28 dBm of RF power, and
the receiver becomes progressively more sensitive as the data rate is lowered. So for
greatest operating range, one of the three lower RF data rates should be used.
The maximum DNT500 HopDuration setting is about 200 ms regardless of the RF data
rate chosen. For a given data rate, FHSS operation tends to become more robust as hop
duration is reduced. However, running with a shorter hop duration may require setting the