Fresenius Medical Care 4008 E Technical Manual page 543

Brightness control (circuit diagram 1/6)
The clock pulses required for the bar graph displays are delivered by IC 74 and the following gate
logic. Superimposed by a higher frequency and a variable mark-to-space ratio, these clock
pulses are used to modulate the brightness. To this end, the shift register IC 6 is provided. IC 6 is
loaded in parallel with the desired mark-to-space ratio via a data word at DB0 to DB7 (CS21).
Status indicator LEDs (circuit diagram 4/6)
All status indicators as well as the condition indicator are switched by means of IC 6 (CS16,
CS17).
Both modules are
– connected to the brightness control by R9,
– not connected to the brightness control by R8 (maximum brightness).
The I/O LED (power on/off) is directly connected to the 5-V supply network.
Alarm indicator LEDs (circuit diagram 5/6)
All alarm indicators are energized by IC 12 and CS18. The key click is controlled by output 19 of
IC 12 via IC 32/5.6.
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Fresenius Medical Care 4008 4/09.03 (TM)