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Control information for A4 IF-Video assembly is transmitted over the Instrument Bus from
the Al5 Controller and decoded by the A4A9 IF Control.
Address Decoder @J
The Address Decoder monitors ADR O-4 and activates the input latches for addresses 20 and
21 when control line LTIO goes low. Address 20 activates input latches UlO, U14, and U17
(U6, U8, and U9 in early versions of A4A9). Address 21 activates input latches U7, U18, and
shows the line levels of the ADR control lines for addresses 20 and 21.
Input Latches @
Input latches hold the IOB levels present at the input of the latch when the address decoder
activates the input latch. Six input latches are activated (clocked) by 2 control lines (Address
20 and Address 21) from the Address Decoder @. For example, when Address 20 goes high,
input latches UlO, U14, and U17 (U6, U8, and U9 in early versions of A4A9) are activated.
The levels of all sixteen IOB lines, IOBO through IOB15, are held at those same levels at the
outputs of UlO, U14, and U17.
The AO.0 to A15.9 dB Control circuitry provides attenuation control voltages for the A4A5
Step Gain. Up to 14 dB of attenuation is accomplished by switching on discrete attenuation
steps on the A465 Step Gain via the A8dB, A4dB, and A2dB control switching lines. The
smaller attenuation steps (0.1 dB through 1.9 dB) are accomplished by controlling the amount
of dc current through a PIN diode attenuator on A4A5. One of the outputs of Ull goes low
when a particular step of attenuation is selected. The output of U19A also goes low when an
odd step of attenuation is desired (0.1 dB, 0.3 dB, 1.5 dB, and so forth). The fixed resistors
tied to edge- connector pin P2-7 determine the amout of current through (and hence the
attenuation of) the PIN diode attenuator. The operation of the attenuator is described in
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