Because RFL™ and Hubbell® have a policy of continuous product improvement, we reserve the right to change designs and specifications without notice.
RFL VF-8A RFL Electronics Inc.
August 6, 2010 16 (973) 334-3100
Voltage regulators step-down the +15V and –15V analog supplies to produce multiple +5V and –5V
analog supplies. The analog +5V and –5V supplies are used to power all analog ICs including the audio
amplifier, stereo volume control, analog switch, DTMF decoder, and the CODEC. Decoupling
capacitors are used as required. The analog +5V supplies can be monitored at test points TP3-5.
The analog -5V supply can be monitored at test point TP6.
All pins which connect to the backplane, except for voltages and grounds, are cut back to prevent
components from being driven by signals before the module is fully powered.
TRANSMIT SECTION
An analog voice input is fed from the MA-306 I/O into the VF-8A module through the rear backplane
connections via TIP and RING inputs. The TIP and RING are protected by a diode bridge and a bi-
directional telecommunication transorb. These voice input signals go through the Subscriber Line
Interface Circuit (SLIC), trans-hybrid circuit, unity-gain amplifier, stereo volume control, and analog
switch. The voice input then feeds two inputs, the PCM CODEC and the DTMF decoder.
The u-law PCM CODEC converts the analog voice into an 8-bit, 64-Kbps digital signal. The digitized
audio signal then goes to the [Actel] Logic Array, where it is processed before being routed to the
appropriate bus, and into the proper timeslot.
The DTMF decoder monitors all tones generated by the local telephone keypad. If a valid DTMF tone
is detected which lasts for the required interval, the STD A clock signal will clock out the BCD
equivalent of the tone detected into the Logic Array. It is necessary to monitor local tones in order for
the supporting DTMF logic to differentiate between “Unique” station and “All” or “Group” call
initiated locally. The “break-in” feature also requires that local tones be monitored.
In addition to the TIP and RING inputs, locally generated tones are transmitted. TONE 0 and TONE 1
are generated by the Logic Array at specific frequencies, and pass through low-pass filters before being
summed by an operational amplifier. An active TX TONE EN signal from the Logic Array applied to
the analog switch selects the tone to be transmitted and not the local voice input signal. The CODEC
and Logic Array process the tone similar to the local voice input. Transmitted tones include ring back
and re-order.
Local “connection/no-connection” and “ringing/not ringing” conditions are also transmitted. A signal
from the SLIC determines whether (Loop-start) loop current is flowing between TIP and RING. In the
on-hook or ringing condition, the loop is open (idle). In the off-hook condition, the loop is
closed (busy) allowing approximately 20ma of current to flow. The “Ring” mode of the SLIC is
programmed by inputs provided by the Logic Array. The Logic Array qualifies the signal to
prevent false off-hook detection when the phone is ringing. When the SLIC and Logic Array
determine that the phone is off-hook, the SLIC is programmed for “Active” mode. Local “ring”
and “connection” states are routed to the appropriate bus and into the proper timeslot.
To Next Page
Loading...