SECTION 29: MODEM APPLICATION INFORMATION
29.1 INTRODUCTION
The RFL 93B MO Modem Module and RFL 93 LC Line Coupler I/O Module form an analog, voice-grade, four-
wire link to a remote RFL 9300. The RFL 93B MO operates over various grades of voice channels. The line
could be as good as a back-to-back connection, or a Type 3002-conditioned line which is typical for voice-grade
telephone channels. The RFL 93B MO modem module will "learn" the condition of the line when a training se-
quence is received from the remote terminal. The training signals are controlled by the RFL 93B CC Communi-
cations Controller Module (Section 13). The RFL 93B MO operates with the CCITT V.29 modulation standard at
a 7.2-Kbps data rate. (See Section 14 of this manual for more information about these modules.)
29.2 TRANSMITTER LEVELS AND WIRING
The RFL 93B MO's transmitter output appears across terminals TB11-1 and TB11-2 on the RFL 93 LC Line
Coupler I/O Module. The nominal output signal level is -3 dBm to +1 dBm, terminated into 600 ohms. The cable
running from the RFL 93 LC module to the communication equipment must be a shielded twisted pair. The
shield must be grounded at both the RFL 9300 and the communication equipment. The wires in the cable
should be 24AWG or larger. Grounding is accomplished at the RFL 9300 by using the ground bus running along
the top of the rear panel. An independent ground bus must also be connected between the communication
equipment and the RFL 9300 to prevent large currents in the shield from melting the cable and disrupting com-
munications. Protection is provided for signals fed back into the transmitter input; voltages in excess of 5 Vrms
are clamped.
29.3 RECEIVER LEVELS AND WIRING
The RFL 93B MO's receiver input is accepted across terminals TB11-3 and TB11-4 on the RFL 93 LC Line Cou-
pler I/O Module. The receiver's input impedance is 600 ohms, which will match most telephone circuits. The ca-
ble connecting the RFL 93 LC and the communication equipment must be a shielded twisted pair, grounded at
both ends. The wires in the cable should be 24AWG or larger. The receiver has an AGC circuit that allows input
signals from 0 dBm to -39 dBm to be accepted. It is recommended that the input level be set at about -16 dBm,
which is the standard level for telephone circuits. Input protection is provided and signals over 5 Vrms are
clamped before going into the modem circuits.
The AGC will accommodate a wide range of signal levels. However, noise is introduced by the modem's input
amplifier at lower input signals. This could increase the S/N ratio to a point where modem performance is af-
fected.
29.4 S/N RATIO VS DEPENDABILITY
The RFL 93B MO acts as an interface between the analog signals received over a voice channel and the digital
signals accepted by the RFL 93B CC Communications Controller Module. The quality of the analog input is ex-
pressed as a signal-to-noise ratio (S/N), and the quality of a digital signal is expressed as a bit error rate (BER).
The relation between S/N and BER depends on the conditioning of the line, the incoming signal level, and the
definition of bandwidths. The RFL 93B CC monitors the validity of received digital messages by testing an at-
tached security code that comes in with each data word. It will stop accepting digital signals if too many mes-
sages are corrupted over a period of time. For a Type 3002 line, a S/N ratio of +14 dB in will yield 100-percent
dependability if the noise is pulsed for the same duration as a fault (60 ms maximum). If the noise on the line
produces a continuous S/N of +16 dB, the RFL 93B CC module will squelch. The BER OK indicator module will
turn off if the S/N is +19.5 dB. The dependability test results are shown in Figure 28-1.
RFL 9300 RFL Electronics Inc.
May 5, 2002 29 - 1 (973) 334-3100
To Next Page
Loading...