Power On Time Delay

A time delay (fast turn off slow turn on) is provided by discrete components (CR13, R20, C9) between the flip-flop (U5) and the MOSFET (Q3) to allow the switching supplies to return to ground potential on occasions when the switch is cycled rapidly. Without this delay, cycling the switch from ON to STANDBY and back to ON might not allow the switching supplies to fall far enough to generate a reliable Power On Reset pulse to the microprocessor. The Power Supply PCB is capable of operating on a 6...

Pattern Generator

The Pattern Generator circuitry is a finite state machine used to set up the various timing signals used by the FSpO2 circuitry. These signals control the phase and rate of the LED drives, the gain and timing of the synchronous detector, and the gating of signal energy into the low-pass analog filters. The heart of the Pattern Generator is an EPROM (U64) capable of supporting a total of eight different timing patterns, each of which is 256 elements long. The addresses for the EPROM are provided...

Block Diagram Analysis

The following discussion analyzes the N-400 major circuit levels using block diagrams. Refer to the overall block diagram on the foldout pages at the end of this section.The circuits and components of the N-400 system are divided into two major functional blocks patient circuits and communication circuits. The terms patient circuits and communications circuits refer to the physical parts of an N-400 system and define the electrical safety characteristics of the monitor. Circuits in the patient...

Processor Chip Selects

The microprocessor has 13 built-in chip select lines LCS , UCS , MCS0-3 , and PCS0-6 all are active low. LCS Lower Chip Select is used to select the RAM memory. Since RAM accesses may be in either bytes or words, separate RAM IC decoding must be employed to avoid corrupting data during byte-wide write cycles. The LCS signal is logically OR'd U68 with address line A0 to generate the chip select signal LOW BYTE SELECT for the even address RAM. The LCS signal is OR'd U68 with the BHE Bus High...