Chapter 1: Functional Description; Control Board - GE Giraffe Service Manual

Chapter 1: Functional Description

This functional description is divided into four sections representing each of the four boards. The reader should
also reference the block diagram and wiring diagram when studying this section.

1.1 Control Board

The Intel 80C188EC microcontroller is an enhanced X86 processor with many on-board peripheral features,
such as a interrupt controller, DMA controller, peripheral chip select driver, programmable timers, etc. The
two programmable timers are used to control the bed heaters. The input to these timers is line frequency.
This allows the control signal to be synchronized with the line frequency to better control the zerocrossing
solid state relays. The on-board interrupt controller has several interrupts: analog-to-digital converter (ADC)
conversion ready signal, overtemperature comparator output, watchdog output, power fail signal, and module
interrupt signal from the system data bus. The microcontroller external bus is a multiplexed address and data
bus.
The system memory consists of a programmable read-only memory (PROM) and static random access memory
(SRAM). The EEPROM is used for calibration and biomedical configuration values. This memory holds the data
even after power is turned off.
The RS-485 integrated circuit converts the RS-232 TTL signals from the microcontroller to RS-485 signals for
the bus. This bus is the main communications bus from the control board to the display board, scale, and servo
oxygen board.
There are two isolation transceivers used to isolate the circuits powered by +5V and the circuits powered by
+5VSTBY (battery backup).
The board contains a 16 channel multiplexer. There are seven temperature measurement channels. These
channels measure the two patient probes and the compartment probe (with two thermistors each), and the
heat sink sensor used to measure the heat sink temperature. Additional channels include the humidity sensor
(RHIN), LINE COMP & LINE COMP2, 5 Volts, Motor current, Vthref, VDAC, and 1.2Vind.
Attached to the compartment probe connection is the relative humidity signal conditioning circuitry. The 1V
reference that is used for the analog circuitry is also the maximum input voltage and the offset voltage for the
ADC. This yields a purely ratiometric system.
The overtemperature circuit compares the air temperature to a reference level, generates an interrupt, and
turns off the heat if the air temperature is higher than the reference level. The overtemperature circuit requires
varying its voltage levels to accommodate various thermistor measurements. This is because the calibration is
digital (no potentiometer).
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