RFL 9300 RFL Electronics Inc.
May 1, 1998 14 - 3 (973) 334-3100
TP6 Monitoring point for TX data received from RFL 93B CC module; this is a digital signal that
varies between zero and 5 volts.
TP7 Monitoring point for TX clock; this is a digital signal that varies between zero and 5 volts at
a rate of 7.2 Kbps.
TP8 Monitoring point for TX OUT; this is an analog signal (+2 dBm nominal).
14.3 THEORY OF OPERATION
14.3.1 OVERVIEW
The RFL 93B MO Modem Module contains a receiver and a transmitter. The receiver converts analog signals into
digital signals, and the transmitter converts digital signals into analog signals. All the digital signals flow between
the RFL 93B CC module and the RFL 93B MO; the RFL 93B MO is also the source of all serial communication
clocks. The RFL 93B CC module has two links with the RFL 93B MO; a synchronous serial data link, and a parallel
control bus. The serial link sends and receives the relay data, while the control bus monitors and commands opera-
tions within the RFL 93B MO.
When the RFL 9300 is initialized, the RFL 93B CC module uses the control bus to configure the RFL 93B MO for
CCITT .29 operation at a 7.2-Kbps data rate. The RFL 93B MO uses a modulation scheme known as "QAM" (quad-
rature amplitude modulation). QAM works by outputting eight tones of different phase and amplitude at a rate of
2400 times per second (2400 baud). Each different tone or "constellation point" (vector) represents a different 3-bit
value. Since points are sent at a rate of 2400 per second, the digital data rate is 7200 bits per second (2400 x 3 =
7200).
The RFL 93B MO must establish a reference point before it can start decoding the data. The reference is estab-
lished when the modem receives a special series of tones called a "training sequence." An internal DSP phase-lock
loop latches onto the received signals, and the RX clock is now phase-locked to the remote TX clock. If the mo-
dems are not locked onto each other, the phase-lock loop will free-wheel. There will be TX and RX clocks, but they
will not be in agreement with the clocks at the remote modem. Under normal conditions, bi-directional communica-
tion is established and both local and remote clock pairs are locked together.
The transmit and receive data is valid during positive clock edges. The RFL 93B MO's AGC circuit allows a wide
dynamic range of received signals (0 dBm to –39 dBm, as measured into the RFL 93 LC I/O module). If the incom-
ing signal level drops below -43 dBm, the INITALRM control line from the RFL 93B MO to the RFL 93B CC module
flags the signal loss.
When the RFL 93B CC module sends the RESET command to the RFL 93B MO through the control bus, the mo-
dem's internal registers are cleared to their default values. The RFL 93B CC module follows up the reset with a se-
ries of commands written into the "interface" memory located in the modem. Before this is completed, there are no
TX or RX clocks coming out of the modem. If there are no clocks coming out of the modem, the problem may be in
the control bus between the RFL 93B CC module and the RFL 93B MO module.
14.3.2 HARDWARE DESCRIPTION
The RFL 93B MO Modem Module contains a plug-on modem card, RS-422 interface devices, reset and address
latch circuits, multiplex control devices, two voltage regulator circuits, and a solid-state heater for the analog con-
troller. Test points are provided for monitoring the RX and TX serial clocks and data, the voltage regulator outputs,
and the RX and TX analog signals from the RFL 93 LC I/O module.
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