GE MAC 5500, Service Manual

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2-26 MAC

5500 resting ECG analysis system Revision E
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2020299-020
Equipment Overview: Theory of Operation
Daughter Board Interface
The interface is realized using a 100 pin high speed connector. This interface
provide two serial interfaces, PC Card interface signals, USB host and various
power supply tappings. The PC Card interface and USB interface are no longer in
the requirement list. All the PC Card signals are buffered. The buffer will be active
only when a valid PC card is inserted in the daughter board. Out of the two serial
interfaces, one provides full hardware handshaking. This is derived from the Super
IO COM2. The COM2 can be routed to either COM2 external connector or to the
daughter board interface using a multiplexer controlled using ATMEL CPU port
pins. The second serial interface has limited hardware control and derived from
ATMEL CPU UART 1.
VGA LCD/CRT Interface
An internal backlit LCD is home for the MAC 5500’s graphical interface. In
addition, external VGA monitors are supported for stress applications. Control for a
standard VGA format (640 x 480 pixels) LC display is provided by the FPGA. The
board is designed to support MAC3500 LCD display also. Though the interface to
LCD is same, external CCFL backlit inverter is different for both display. Two
connectors are provided for external CCFL backlight inverters as well as two digital
controls for On/Off and brightness. While the FPGA is capable of directly driving
the LCD, external hardware is required to generate the analog video levels expected
by external VGA monitors.
LCD Panel EMI Reduction Components
To reduce EMI, 47pF capacitors have been added to all LCD digital lines. In
addition, 49.9

resistors have been added to the video clock and Sync lines.
CRT Video DAC / Sync / Buffers
A triple 6-bit video DAC supports external analog VGA monitors. Only one DAC/
Level Shifter/Buffer will be described, as they are all identical in function. The
video output is referenced to a filtered tap (FB107, C29) off the +3V-M supply rail
and then level shifted back to ground.
Each DAC is comprised of six binary weighted resistors and a seventh blank/sync
signal resistor. The FPGA LCD data outputs sink current through the 75

load
resistor in proportion to their respective DAC resistors. The voltage across the 75

load resistor represents the sum of all drive currents. Minor non-linearity is
introduced in the DAC transfer function by the fact that the summing junction varies
in voltage with DAC current.
The 3.3V referred video is shifted back to ground by a blocking capacitor. The
shifted video signal is buffered (and further shifted) by emitter followers.
Transistors clamp the negative excursions of the bases of the emitter followers to
one diode drop above ground, so the most negative level at the emitter of the emitter
followers is ground. Nominal full-scale swing is 1VP-P (blank to white).
Bias for the base of the clamp transistors is provided by a 1.4V bias supply
consisting of a stack of two diode connected transistors (Q8). This 2Vbe bias exactly
cancels the 2Vbe shift produced by the level clamp and output buffer. Since all