Sierra Video Tahoe Series 16 - Manual (Page 52)

Sierra Video Systems
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impedance output to sync separator Q1. The output of Q1 is inverted sync. U23E and
U23F form a low pass filter which separates vertical sync from the signal. U23D performs
several functions. When sync is present at its input, it acts as an inverter. If no sync is
present at the output of U23E, it may be in either state it is AC coupled by C1 to U23D.
U23D then functions as a free running oscillator. The oscillator is made by the resistive
feedback around U23D and the value of C1. The oscillator does not function when sync
is present because the input to U23D is driven by U23E to the full 5V input range. This
overdrives the oscillator mode which uses the much lower (about 1V) hysteresis range of
U23. U23D’s output drives the DREQ input of the MC68302.
Serial Interfaces
The RS485 control panel network is interfaced to by U16, an EIA RS485 transceiver. The
RS485 network is terminated on the 504001 by a complex network. R101 provides a
simple resistive load. R103 in conjunction with D15 and D16 provides a lower impedance
termination as the peak to peak swing across the line exceeds the forward drop of the
diodes. C26 was selected to reduce ringing. This complex termination performs much
better than the usual single 100 ohm resistor used with RS485 lines. It was optimized for
31250 rate and Belden 8451 cable. At 31250 baud, 64 panels can operate over a
combined cable length of 2000 ft. At 9600 baud, the cable total length can be up to 5000
ft.
Power Supply
Most of the circuits on the 504001 are powered by +5VDC. The RS232 interface circuits
use + and -12VDC. Low voltage AC enters the module. It is then rectified and filtered.
Three terminal fixed voltage regulators are used to provide the required ages.
Video Crosspoint Module 504044
The 504044, 16x16 video crosspoint module is a plug-in module that is a complete video
switching module containing; input buffers, crosspoint array, output amplifiers, control
logic and power supply. It is the basic building block for the video portion of the Tahoe
Series 16 routing switcher. The 504044 is available in a standard bandwidth of 50 MHz.
Input buffers
Each of the 16 video inputs is coupled to a 480 MHZ wide band video buffer
(U4). R1 provides a DC reference to the input of the buffer. R22 and C35 form a
snubber network which compensates for the phase shift loading effect the
crosspoint buses have on the output of the buffer.
Crosspoint Matrix
The crosspoints on the 504044 are configured from individual 4 by 1 IC crosspoint chips.
Each of the eight buses has four 4 by 1 IC’s on it. Each switching IC has 3 control inputs.
The “A” and “B” inputs select “1 of 4” inputs for that IC. The third control input is a CS
(chip select). By using the chip select input, multiple 4 by 1 IC’s can be linked to form a
larger bus.
The output of each crosspoint chip passes through a PIN diode (D9) before being bussed
together. The PIN diode blocks the off capacitance of the 7 deselected chips and has an
extremely low forward impedance. The common bus of eight diodes has an FET (Q1)
configured as a constant current source connected as a load for the bus.
Output Amplifiers
The output amplifier circuit is based on a current mode feedback amplifier IC. The
standard 504044 uses an EL2020 (Elantec) or CLC430 (Comlinear) for this function. R5