Table of Contents
Chapter 1: Functional Description
to provide a 2500 VRMS isolated serial interface to meet the logic levels specified by EIA RS-232D and
CCITTV.28.
The MAX250 and MAX251 (U1 and U2), together with two 6N136 optocouplers and transformer TR1, form
an isolated RS-232 transmitter and receiver. The MAX250 connects to the non-isolated or "logic" side of the
interface, translating logic signals to and from the optocouplers, while the MAX251 resides on the isolated or
"cable" side, translating data between the optocouplers and RS-232 line drivers and receivers. In addition to
the optocoupler drivers and receivers, the MAX250 also contains isolation transformer drive circuitry which
supplies power to the isolated side of the interface, and the MAX251.
The transmit signal is input to the MAX250 driver (U1 pin 4) whose output (U1 pin 3) drives optocoupler U4.
The optocoupler output (U4 pin 6) is then fed into the MAX251 driver (U2 pin 3). The output of the MAX251
driver (U2 pin 12) is at the logic levels conforming to EIA RS-232D and CCITTV.28. Conversely, the receive signal
enters the MAX251 driver (U2 pin 10) and is stepped down to CMOS/TTL levels at U2 pin 5. This logic level drives
optoisolator input (U3 pin 3) whose output is fed into U1 pin 10. The output (U1 pin 9) signal is then available to
the control printed circuit board.
A slide switch SW1 is used as a "self test" for the RS-232 interface. In the closed position, the J30-1 transmit
signal is sent through the MAX250/MAX251 transmitter and back into the receiver portions. The signal can be
read at J30-2 and verified to be correct. Any external cable connection must be removed for this self test to
function. CR1 and CR2 provide transient protection for MAX251. In normal operation SW1 should be in the open
(OFF) position.
The nurse call signal is input at J30-5 as a TTL logic level. In the "no alarm" state, this signal is a logic high,
which turns on Darlington Q1, energizing relay K1. This results in contact closure between J31-1 and J31-2.
In the "alarm" state, J30-5 is a logic low, which turns off Q1, de-energizes K1 and results in contact closure
between J31-2 and J31-3. K1 provides 2500 VRMS isolation between the relay coil inputs and contact outputs.
1.8 Servo Controlled Oxygen Option
The Giraffe Servo Control Oxygen System consists of an oxygen sensing circuit, Servo Oxygen circuit board, and
an oxygen delivery system.
The sensing circuit is located beneath the bed and consists of a pair of fuel cell oxygen sensors, a three-way
solenoid calibration valve, and a calibration fan. In normal operation the calibration valve is closed and allows
the Giraffe fan to circulate gas from the infant compartment across the sensors.
The unit must be calibrated at least every 24 hours when servo oxygen is in use. After 24 hours have elapsed
the system prompts the user to perform calibration. Once the operator initiates calibration, the calibration
valve opens and the calibration fan is turned on. This draws ambient air across the sensors until a stable
reading is obtained. This 21% oxygen reference value is then used to calibrate the measuring algorithm. After
calibration 100% oxygen is briefly delivered to the system to ensure there are no occlusions. When calibration
is complete the unit will resume controlling oxygen based on the last set point.
The system must have two sensors present to operate. One sensor is always used for control and the other is
used for a redundant check and display. The sensors generate a voltage of about 40 millivolts at 21% oxygen
concentration and about 200 millivolts at 100% oxygen concentration. The voltage is directly proportional
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Troubleshooting
