DNT500
2008 by RF Monolithics,
Inc.
11
M-0500-0000 Rev D
BaseSlotSize and RemoteSlotSize parameters well below their maximum values at the
lower RF data rates. The equation below calculates the minimum hop duration needed at
a given RF data rate for a specific number of remote slots and BaseSlotSize and Re-
moteSlotSize parameter settings. Support for optimizing a DNT500 configuration for a
specific application is also available from RFM’s Technical Support Group. See Section
10.3. for technical support contact information.
The minimum required hop duration for a DNT500 configuration is:
T
HD
= T
BRO
+ N
RS
*T
RO
+ T
RFB
*(B
BSS
+ N
RS
*B
RSS
)
Where:
T
HD
is the minimum required hop duration in milliseconds
T
BRO
is the base and registration request overhead time for each hop (RF data rate dependent)
N
RS
is the number of remote slots
T
RO
is the remote overhead time for each hop (RF data rate dependent)
T
RFB
is the transmission time for one user byte (RF data rate dependent)
B
BSS
is
the
BaseSlotSize parameter in bytes
B
RSS
is
the
RemoteSlotSize parameter in bytes
The constants in the equation for each RF data rate are given in the following table:
RF Data Rate
kb/s
T
BRO
ms
T
RO
ms
T
RFB
ms
38.40 11.620 4.817 0.2080
115.2 4.953 2.039 0.0694
200 3.540 1.450
0.0400
500 2.388 0.970
0.0160
For Example 1, consider a point-to-point CSMA Mode 0 system operating at 38.4 kb/s
with the BaseSlotSize parameter set to 133 bytes and the RemoteSlotSize parameter set to
128 bytes. The minimum hop duration needed to support one-hop transmissions of full
slot size messages in both directions for this configuration is:
= 11.620 + 1*4.817 + 0.2080*(133 + 1*128)
= 16.437 + 0.2080*261
= 70.725 ms
The closest programmable hop duration is 70.750 ms.
It should be noted that the base station operating system will commandeer 5 bytes from
the BaseSlotSize allocation in Mode 0 and up to 13 bytes in Mode 1 to send reception ac-
knowledgements (ACKs) back to the remotes. The BaseSlotSize should be sized accord-
ingly. In the above example, the BaseSlotSize parameter is set five bytes larger than the
RemoteSlotSize parameter to accommodate the ACK bytes.
When running a point-to-multipoint network with uncoordinated remotes using CSMA
Mode 1, it is useful to set N
RS
to a value of 3 or higher in the equation. Although CSMA