Commercial In Confidence
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zero, it is only of use where a controller makes use of the DCD signal to suppress data
processing but suffers some delay in processing received data.
3.16.4.1
RECEIVE TO TRANSMIT SWITCHING TIME
When using the internal modem the action that initiates transmission can be either receipt of
a character at the serial port or the operation of RTS. These examples use the first mode. The
radio does nothing until the stop bit of the first character for transmission has been received,
the transmitter is then started:
The time delay between receipt of the stop bit for the first character to be transmitted at the
transmitting radio and output of the start bit of that character at the receiving radio is the
sum of the values ttxon, tlid, trbyte, and tmdel shown in the diagram above. Values for these
parameters are indicated below:
TABLE A:
Timing values for duplex and simplex modes are as follows:
symbol description
duplex
simplex
ttxon
Time from external action to commencing transmission
1.3ms
9ms
tlid
Duration of synchronisation transmission (lead in delay)
Table B
Table B
trbyte
Duration of 1 byte at radio signal baud rate
Table C
Table C
tmdel
Modem decode latency
Table D
Table D
TABLE B:
The lead in delay is a programmable parameter but minimum values dependant
on baud rate must be adhered to. However, in a scanning system with the base station on
continuos transmit the base station lead in delay can be set for Zero (thereby saving valuable
time) as the internal outstation modems will always be synchronised.
Baud 150 300 600 1200 2400 4800 9600
Min
tlid
80ms 60ms 40ms 40ms 40ms 20ms 20ms
TABLE C:
The duration of a byte at the radio baud rate is dependant upon the data format
employed, the table below assumes a format of one start bit, 8 data bits, no parity and 1 stop
bit, i.e. a total of 10 bits per character. If another format is used the appropriate correction
must be made.
Baud
150 300 600 1200 2400 4800 9600
trbyte 66.7ms 33.3ms 16.7ms 8.3ms 4.17ms 2.08ms 1.04ms
TABLE D:
The modem decode latency takes into account delays introduced by hardware and
software filters. The total delay is baud rate dependant:
Baud
150 300 600 1200 2400 4800 9600
tmdel 6.9ms 3.5ms 1.7ms 1.3ms 1ms 1ms 1ms
