DNT500
2008 by RF Monolithics,
Inc.
13
M-0500-0000 Rev D
there are overheads related to hopping frequencies, assembling packets from the serial
port data stream, transmitting them, sending ACK’s to confirm error-free reception, and
occasional transmission retries when errors occur.
For Example 3, consider a CSMA Mode 0 transparent data system operating at 500 kb/s
with the BaseSlotSize parameter set to 133 bytes (128 bytes net after the five byte alloca-
tion for sending ACKs) and the RemoteSlotSize parameters set to 128 bytes. The mini-
mum hop duration needed to efficiently support this configuration is:
= 2.388 + 1*0.970 + 0.0160*(133 +1*128)
= 3.358 + 0.0160*261
= 7.534 ms
Setting the hop duration to 7.55 ms, the average full-duplex serial port byte rate that can
be supported under error free conditions is:
128 Bytes /7.55 ms = 16.942 kB/s, or 169.42 kb/s for 8N1
Continuous full-duplex serial port data streams at a baud rate of 115.2 k/bs can be sup-
ported by this configuration, provided only occasional RF transmission errors occur. Plan
on an average serial port data flow of 75% of the calculated error-free capacity for gen-
eral-purpose applications, and 50% of the calculated error-free capacity for RF challeng-
ing applications such as vehicle telemetry and heavy industrial process environments.
Most applications do not require continuous serial port data flow. The DNT500 transmit
and receive buffers hold at least 1024 bytes and will accept brief bursts of data at high
baud rates, provided the average serial port data flow such as shown in Example 3 is not
exceeded. It is strongly recommended that the DNT500 host use hardware flow control.
The host must send no more than 32 bytes additional bytes to the DNT500 when the
DNT500 de-asserts the hosts CTS line. In turn, the DNT500 will send no more than one
byte following the host de-asserting its RTS line. Three-wire serial port operation is al-
lowed by connecting the DNT500 CTS output to its RTS input. However, three-wire op-
eration should be limited to applications that send small bursts of data occasionally at an
average serial port data flow less than 50% of the calculated error-free capacity. Data loss
is possible under adverse RF channel conditions when using three-wire serial operation.
2.7.5 Sleep Mode
To save power in applications where a remote transmits infrequently, the DNT500 sup-
ports a Sleep Mode. Sleep Mode is entered by switching DTR Pin 11 on the DNT500
from logic low to high. While in Sleep Mode, the DNT500 consumes less than 0.5 mA.
This mode allows a DNT500 to be powered off while its host device remains powered.
After leaving Sleep Mode (Pin 11 low to high), the radio must re-synchronize with the
base station and re-register.