Sierra Video Manzanita 161VS - Manual (Page 7)

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Sierra Video Systems • [email protected] • http://www.sierravideo.com
MODEL 161 INSTRUCTION MANUAL
Incandescent control panel.
The 803139 is a 16 switch assembly which uses
incandescent lamps and includes switch encoding
and lamp driver circuits. The 803138 is primarily
used with the Model 161, 16 by 1 video plus
stereo audio switcher.
Refer to the schematic diagram while reading the
following circuit description. The 16 switches are
connected together in a 4 by 4 matrix configura-
tion. This allows U1, a keypad encoder, to be
used to convert the one-of-sixteen to a four bit
binary code. U1 also provides switch debounce
and key rollover which means that if two buttons
are both selected, only the first switch detected is
encoded until all switches are released. When no
switch is selected the output of U1 is a high
impedance. The latching circuit on the 503135
(the other end of the system), is a 23K ohm source
impedance. This medium impedance holds the
four data lines at either 0 or +12V. When a
switch is pushed the DA output of U1 is inverted
by Q1 and puts a low on the OE input of U1
causing the output of U1 to become a low imped-
ance. This forces the 503135 logic to follow and
hold the selected four bit code.
The lamp circuit of the 803139 consists of U2, a
dual two line to four line decoder. The first half
of U2's outputs are current boosted by emitter
followers Q2 through Q5. Q6 through Q9 invert
the remaining outputs of U2. Q2 through Q9
connect to the incandescent lamps in the switches
in a four by four matrix fashion.
The 803139 is powered with unregulated DC. U3
provides regulated +12V for U1 and U2. A 47
ohm resistor limits in rush current to lamp driving
transistors Q2 through Q5.