Smiths Medical BCI 3304, Service Manual

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Chapter 2: Oximeter Circuit Description
2-4 3304 Service Manual
Interface Circuit
Addressable Latch and Speaker Output
Addressable latch U21 and U22 decodes A0-A7 and provides 16 outputs
for controlling the oximeter board’s circuitry. Four outputs, pins 5, 6, 7
and 9 of U22 create a 16 step D/A converter which defines the speaker
volume. Transistor Q5 drives the speaker under control of the TONE
signal. Addressable latch U29 generates two more output signals to
control LED drives.
Input buffer
Input buffer U20 allows microprocessor to read signals from both
keypads and signals from the analog section.
Synchronous Serial Port
The microprocessor’s synchronous serial port TXS/CKS is used to send
data to the digital potentiometer U6 and to send data to the display
driver chips U1 and U2 (on the display board). It is also used to read
data from A/D converter U8. U6’s data is latched with the I/O strobe
POT-LD; display driver chips U1’s and U2’s data is latched with the I/O
strobe DISP-LD.
Asynchronous Serial Port
The microprocessor has two internal asynchronous serial ports:
TXA0/RXA0 and TXA1/RXA1. TXA1/RXA1 is used for manufacturer test
applications. TXA0/RXA0 is used for sending printer data to the printer
through isolation barrier.
RS-232 Isolation Interface
TXA0 is buffered by U18E and controls the LED of the opto coupler
ISO1 through transistor switch Q6. The output of ISO1 controls the
RS-232 driver U24 which generates both positive and negative voltage
levels required by RS-232 interface. The /BUSY signal from the RS-
232 connector is converted to CMOS level by U24 and controls LED
of opto coupler ISO2 to transfer data or a busy signal back to the
microprocessor. The second receive - transmit channel of U24 is used
in the production test mode. In this case J5 and J6 are connected
together temporarily breaking the isolation barrier, J7 is used to
communicate to a personal computer.