ZRT Manual
Page 20 of 37
Rev. J – 11 September 2008
5.13.2 Handshaking on RS232-Only (Full Handshaking) and TTL versions
The RS232 Only (Full Handshaking) and TTL versions can be programmed either to use
RTS/CTS handshaking to initiate transmission, or to transmit automatically whenever data is
present at the serial input. In the latter mode CTS is still operated to implement flow control but
can be ignored unless message sizes exceed 1k byte and the serial port baud rate is higher than
the radio signal baud rate. These handshaking modes are compatible with modes A, C and D of
the CMD400 manufactured by Pacscom Ltd.. Mode B (byte stuffing mode) is not supported.
Transmission Using RTS/CTS Handshaking (RS232-Only and TTL Versions):-
If handshaking is enabled, transmission is started by operating RTS. CTS can then be monitored
for flow control purposes. In the idle state CTS is inactive, but when RTS is operated CTS will
become active immediately and data may be input to the serial port. When all data has been
loaded to the serial port RTS should be dropped. Transmission will continue until all data in
the serial input buffer has been sent, then CTS will become inactive and transmission will cease.
During transmission the amount of data in the serial buffer is checked by the radio, if the buffer
becomes ¾ full CTS is dropped to request the host to stop loading data, CTS is activated again
when the buffer is reduced to ¼ full. To prevent timing problems data will still be accepted into
the buffer when CTS is de-activated due to buffer filling during transmit, however any data
received once CTS has dropped at the end of a transmission will be discarded, this prevents
such data from being prefixed to the beginning of the next message.
Transmission Without Hardware Handshaking (RS232-Only and TTL Versions):-
If RTS/CTS handshaking is disabled the radio will start transmission as soon as data is received
at the serial port, transmission ceases as soon as the serial buffer has been emptied and a period
equivalent to two characters at the radio signal baud rate has elapsed. It is important to note
that since transmission ceases as soon as a two character delay in the incoming data stream is
seen, data characters in a message must be presented in a continuous back to back stream.
In this mode CTS is still used to indicate the serial buffer fill level in the same way as described
in the section on transmission using handshake, the difference is that in the idle state CTS is
always active indicating readiness to accept data. In most applications CTS can be ignored as
messages are likely to be smaller than the serial input buffer (1k byte), bear in mind also that if
the radio baud rate and data format is the same as that configured for the serial port the buffer
is being emptied as fast as it is being filled and so buffer overrun is unlikely.
5.13.3 Data Reception
Any data received by the radio is simply output to the serial port, and in RS232 configurations
the DCD line can be programmed to operate in three different modes to assist the host. Firstly
by indicating that a carrier is detected on the radio channel, this is useful if a busy lockout
function is required (although this can be dangerous if the channel is susceptible to interference
as well as wanted signals), secondly DCD can indicate presence of a carrier and a valid data
signal, data will normally be output under this circumstance, the third mode behaves in the
same way as the second except that DCD remains active until all data has been output to the
serial port after the signal has gone, this allows DCD to be used as a wake up signal.
In RS422 and RS485 2-wire configurations, the radio will output data on to the 2 wire circuit
whenever it is received, which could lead to a bus conflict in conditions of high interference,
preventing any connected terminal from transmitting data when it wants to. To avoid this
condition it is recommended that the message packeting option is turned on at both ends of the
radio link in this mode.
