Mindray Passport 5-Lead, Service Manual

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Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 65
Theory of Operation Block Diagrams
The vertical video counter is used to count the 262 rows of the display format (240 of which
are displayed, the remaining are used for vertical retrace.) The vertical video counter is
clocked by the rising edge of the input horizontal sync and cleared by the input vertical sync
signal. The vertical video counter wraps at count 120, which demarcates the upper and
lower halves of the screen. This is used to toggle the signal TOP/, which indicates which half
of the screen the video data is in. The vertical counter is used to drive the row address of the
VRAM.
Because of the timing requirements of the EL panel, the vertical line timing of the output video
need to be different than the input line timing. To accommodate this, a separate horizontal
and vertical output counters are used to determine when the appropriate row address should
be strobed into the VRAM to initiate another row of output video data. After this row address
is strobed in the VRAM by the RAS/ signal, the CAS/ signal strobe in the column start tap
point, which, in this case, is always zero. An address multiplexer is used to select the
appropriate input and output row and column counters respectively.
The EL panel timing is such that the input video clock of 14.318MHz is divided by 2.5 to
generate the output video clock. A new vertical sync is generated at 120Hz, as well as a
new horizontal sync.
EL and Color LCD Interface
The EL and Color LCD Panel Boards (670-00-0617-XX) has a built-in frame buffer and the
interface is regular timing at a 60Hz rate. Therefore, the interface requirements are much
simpler. All that is necessary is each input horizontal scan line needs to be doubled to
convert the 640x240 input format to the 640x480 output format.
A 910x8 bit video line memory buffer is used to temporarily store each incoming video line
and clocked out twice as fast to double the video line. The input video clock of 14.318MHz
is doubled by a clock doubler circuit. The input horizontal sync signal is doubled simply by
feeding it to the video line buffer and clocking out twice as fast. The RGBI input color video
signals are also fed directly into the video line buffer and clocked out twice as fast directly to
the EL or LCD panel.
The main function of the PAL is to fit the 912 input columns into the 910 bit video line buffer
by effectively deleting two input clock cycles. It also generates miscellaneous control signals
required by the video line buffer.
Battery Charging LED
The EL or Color LCD Panel Board controls the Battery Charging LED (Color: Green). The LED
is lit when the batteries are charging no matter what position the power switch on the
Passport is in. Two comparators are use to compare the Battery Charging voltage to the
Battery voltage. When the charging voltage is greater than the battery voltage, the output of
the comparators is low which turns on the LED. The function is powered by an ex17 V
which improves the internal power management and allows the system only to work when
the external power supply is plugged in.