1892 1139 - RSX Range of Intelligent Modems User Guide - v7.1 / Nov 2007
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4.3
TIMING
The modem system is “intelligent” and removes the need for the user to understand how
the radio system operates in order to use the modem.
The data to be transmitted is automatically stored while the transmitter is enabled A
preamble sequence is then transmitted to ensure that the receive modem can synchronise
to the incoming signal before the data is transmitted. The data is placed in HDLC packets
with header and CRC information added.
This procedure introduces a delay to the through packet of data which depends on the
baud rates, the type of radio and the size of the input packets. The following table
overleaf gives timing information for the RSX450 using the SX450G transceiver. For the
tests, the following set-up was used:
Input/Output RS232 baud rate
-
As stated in table
Parity
-
Even
Data bits
-
8
CRC check
-
On
Packet Size
-
128
RF Baud Rate Packet Size (bytes) I/P-O/P Delay (ms) Total I/P-O/P Time (ms) Aggregate Baud Rate
RS232 baud rate = 9600
9600
42
80
130
3554
9600
332
125
540
6763
9600
2402
125
2950
8957
4800
42
141
191
2419
4800
332
283
890
4103
4800
2402
283
5235
5047
RS232 baud rate = 19200
9600
42
80
103
4485
9600
332
150
452
8080
9600
2402
150
2563
10309
NOTES :
1. The Input-Output delay is measured from the first start bit of the input RS232 data at
the transmitter to the first start bit of the RS232 data output at the receiver.
2. The Total I/P-O/P time is measured from the first start bit of the input RS232 data at
the transmitter to the last stop bit of the output RS232 data at the receiver.
3. The aggregate baud rate is calculated from the total I/P-O/P time and the number of
bytes sent (at 10 bits per byte).
4. Over the RF link, the original data formatted start and stop bits are stripped off which
is why the aggregate baud rate over the radio link can be higher than 9600.
5. There is a minimum time from the end of the receive data being output before the
transmit data can be input to the modem, of 10ms. Any data input to the modem
before this delay may be lost as the modem is still processing the receive data packet.
