ZRT Manual
Page 19 of 37
Rev. J – 11 September 2008
If compatibility with other radios is not required, the use of the synchronous mode is
recommended, as this will give best link performance.
5.10
RADIO DATA FORMATS
The data rate over the air can be set up independently of the rate set for the serial interface, but
the over-air rate should be set either at the same speed or a lower speed than the serial interface
rate. The radio baud rate should be set at the minimum possible to maintain the required
throughput, lower speeds will give better results in poor signal conditions
The radio signal can be set up to operate using 7 or 8 bit data, 1 or 2 stop bits, and odd, even or
no parity. This setting is also independent of the serial port setup. This flexibility allows
compatibility with other radios.
If the Forward Error Corrector is enabled (option only available at 9,600bps), the selected radio
signal format is over-ridden as detailed below.
5.11
FORWARD ERROR CORRECTOR
At 9600bps there is also a programmable option to switch in a forward error corrector. When
switched on, the over-air data format changes to a fixed format using 14 bit words. These
comprise 8 data bits, 5 CRC (Cyclic Redundancy Check) bits and a flag bit which is used to
differentiate control and data functions in messages. An additional 14 bit synchronisation word
is also sent after every 8 data words. The effect of this redundancy on a typical 9600bps link
configuration is to reduce the effective data transfer rate to around 6300bps.
The error corrector is aimed at improving performance in weak signal conditions, rather than
recovering data in deep fades or burst-error conditions. An error rate of 1x10
-4
with the FEC
switched off will typically improve by a factor of 2000 to around 5 in 10
-7
when it is switched on,
but an initial error rate of 1x10
-3
with it off will only improve by a factor of around 250 to
something like 4x10
-5
when it is switched on.
In terms of receiver sensitivity, the 1x10
-6
threshold improves by around 0.4uV (or 6.4dB) when
the FEC is switched on.
5.12
SQUELCH TAIL (DRIBBLE BITS) ELIMINATION
The ZRT has an optional packetisation mode which can be enabled using the configuration
program. This adds framing characters at the start and end of the user’s message prior to
transmission. The additional information is stripped off the messages at the receiver prior to
passing the data to the interface connector. Packetisation can be useful in getting rid of any
spurious characters which may otherwise be generated at the end of messages by squelch noise
as the receiver mutes or by interference and which can affect old or non-tolerant protocols.
It is important to note that packetisation must be set the same on all radios operating together.
All radios must have it selected or all radios must have it de-selected.
5.13
SERIAL INTERFACE & HANDSHAKING
5.13.1 Handshaking on RS232/RS422/RS485 version
In this version of the ZRT, transmission is automatic when transmit data is applied.
When configured for RS232 mode, the RTS line is looped back to CTS and the DSR line is
looped back to DSR. The radio should in most circumstances operate correctly in applications
requiring RTS/CTS handshake although the it is unable to use CTS to prevent buffer overflow.
Overflow situations can easily be avoided by making the serial port baud rate the same as the
radio signal baud rate, or by ensuring that message sizes do not exceed the buffer size of 1024
bytes.
