Omega A2400 - Manual (Page 16)

Functional Description 2-7
T1
As soon as the A2400 detects a character in the RS-485 receive buffer, time
delay T1 is activated. This is a dead time to allow the host to prepare for the
receipt of a message. This is particularly important when a simplex connec-
tion is used, where the send and receive data is transmitted on the same
frequency. During this time the A2400 creates no control output, but any
data received on the RS-485 port is stored in the receive buffer. At the end
of time T1, the A2400 asserts the RTS control signal to ‘key’ or turn on the
transmitter of the radio.
T2
Once the RTS signal has been asserted, the T2 delay is activated. This is
a delay time to allow the transmitter to power up and settle in anticipation of
a transmission back to the host. The settling time required is specified by the
modem manufacturer. When the T2 time period is over, the A2400 will start
to transmit the data held in the receive buffer, and will continue to transmit
until the buffer is empty.
Some radio modems provide a CTS (Clear To Send) signal that indicates
that the transmitter has settled and is ready for data. This signal may be
connected to the CTS input of the A2400 to provide hardware handshaking.
The delay period T2 ends when either the CTS signal is detected or the T2
timer ends, whichever comes first.
T3
The A2400 will transmit data to the modem until the receive buffer is empty.
When the receive buffer is empty, and the last character has been transmit-
ted, time delay T3 is activated. T3 provides two functions: it provides a clean
break between the transmitted data and the turn-off of the radio transmitter,
and it allows the host to poll more data without keying the transmitter on and
off. During time T3, the host may transmit another command which would
pass through the A2400 to the RS-485 bus. Typically, this command would
generate a response from a device on the RS-485 bus. If the response data
is received by the A2400 before T3 is complete, T3 is canceled and the
received data is immediately transferred to the modem. When the receive
buffer is empty, T3 is activated again, and the cycle repeats itself. This allows
the host to establish communications with the remote radio and talk back
and forth without wasting time re-keying the transmitter for each response.
If T3 times out and the receive buffer is empty, the radio connection is
terminated by turning off the RTS signal.