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DNT90 Integration Guide - 05/10/12
can send two messages per hop when the RSS is 73 bytes, provided the total payload bytes in the two
messages is 64 bytes or less:
slot size
= 73
buffer
= 73 + 9 = 82
2 headers
= 2*9 = 18
net for payload = 82 - 18 = 64
Note that the RSS is
calculated
by all DNT90s in a system, rather than being a user configured parame-
ter.
The slot size depends on the current values of the following parameters:
- base slot size
- hop duration
- number of slots in a frame
The system must be configured such that the RSS is big enough to hold the longest protocol message a
remote will send. This is done by setting the appropriate hop duration for the chosen BSS and number of
slots. The required hop duration for a specific number of slots, base slot size and remote slot size is cal-
culated as follows:
HD
hop duration in µs
NS
number of slots
BSS
base slot size in bytes
RSS
remote slot size in bytes
HD = NS*(80*RSS + 2440) + 80*BSS + 3280 (round HD up to an even multiple of 500 µs)
Example:
NS = 4
BSS = 96
RSS = 109
HD = 4*(80*109 + 2440) + 80*96 + 3280
HD = 44640 + 7680 + 3280
HD = 55600 round to 56000 µs = 56 ms
Excel
Formatted Equations (load the Excel analysis ToolPak add-in for the QUOTIENT function):
A B C
D
E
1
Slots
BSS
RSS
Hop Duration in µs
Hop Duration in ms, Rounded
2
Up to the next 0.5 ms Step
3
1
20
20
=A3*(80*C3+2440) + 80*B3 + 3280
=0.5*QUOTIENT((D3+499),500)
For transparent serial port operation without using hardware flow control, the BSS and RSS must be large
enough to accommodate all message bytes that can accumulate between transmissions. The required
BSS and RSS for protocol-formatted messages sent over the wireless link are shown in Table 7.3.1. For
example, the BSS and RSS size required for a
TxData
protocol-formatted message is three bytes less
than the value in the length byte field of the formatted message.
The default BSS is 40 bytes, number of slots is 3 and hop duration is 20 ms. These parameter settings
provide a 25 byte RSS. These default settings are suitable for point-to-point and small to medium point-
to-multipoint systems operating with protocol-formatted and/or transparent messages. To accommodate