4xx Preliminary Generic User Guide. 4 Jan 06
5
The Response
1. The Controller recognizes its address and prepares a message to be sent
back to the TMC computer. Our response message will be sent back to the
TMC on the Receive Pair of the 4wire system.
2. The Controller (Remote) signals the modem that it wants to send data by
raising Request To Send (RTS) at the modems input. The modem returns a
Clear To Send (CTS) back to the Controller after a time delay (user
selectable). This is called Handshaking.
As soon as the “remote” modem sees RTS it transmits a Mark Tone down
the copper wire to wake up the “Master” modem.
3. The TMC Modem’s Carrier Detect (CD) circuit sees the Mark tone. If the
tone is continuous for 4 or 8ms (user selectable) the Carrier Detect circuit
will validate it and unlock the receivers output (removes receiver squelch)
and raise CD at the data port. This CD signal advises the TMC computer
to expect data. The demodulated Mark tone, logic 0, is passed to the
RS232 Data port, where it does not change state due to the fact that when
not in use it is always at logic 0, same as the Controller end. Now let’s go
back to the Controller end.
4. Once the Controller receives CTS (clear to send) it will send data to the
modem. At this juncture both modems on the system are in a “go” state.
5. The Controller now sends data (0’s and 1’s) to the modem which converts
it to Mark and Space frequencies. These frequencies (tones) are then
transmitted down the receive pair of wires to the TMC modems receiver.
6. At the TMC modems receiver the Mark and Space frequencies are
demodulated into 0’s and 1’s respectively. This is the same bit pattern that
the Controller sent to the modem for transmission. This data is then sent to
the TMC computer via the communication port.
When the data ends, RTS is lowered and the modem transmits a short
burst of Soft Carrier (tone is out of band of the Mark/Space frequencies).
This lower frequency tone causes the receiving modem to squelch the
input to the digital circuits. This action “closes the modems door” to
prevent noisy lines from false triggering the data circuits. The modem is
now quiet.
7. We have now completed the Response phase.
Remote
Controller
Transmit
Section
Receive
Section
TMC
Computer
Receive Pair
CD
DATA
RTS
DATA
CTS