Mindray Passport 5-Lead, Service Manual

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Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 23
Theory of Operation Block Diagrams
Port B Function
The inputs in port B are read each time a data pocket is sent by the microcontroller, at a
nominal 286 Hz rate. Not all of the signals to be sampled are at proper CMOS logic levels.
Therefore, a level shift network is employed. This consists of a 47K resistor network RN2 and
CMOS inverting buffer U36. The series resistance of RN2 allows this protection network to
clip the input signals, which may swing between the 7.5 volt rails, to a level acceptable to
the buffer. The input protection network will then carry a current of about 150 A, which is
well below the level needed to induce latching. The output of the buffer is then a clean 0 to 5
volt swing.
The software inverts the data at port B, to compensate for the inversion of the level shifter. The
bits are then transmitted in raw form in a data byte known as STATUS within each packet.
The E-trig signal is additionally subject to special processing. The software detects a rising
edge of this signal and reports it in a separate bit in STATUS.
Port C Function
All bits in port C are configured as outputs. The upper three bits support control of the
respiration function. When the microcontroller receives a respiration mode command, the
upper three of the four data bits associated with the command are asserted on the upper
three lines of port C. These three lines control analog switches in the respiration circuit
providing the off/on, reset and filter control functions. The lower two lines of port C are used
as part of the A/D converter interface, and will be discussed in conjunction with the SPI.
Asynchronous Serial Port (SCI)
The TDO and RDI lines in port D provide the asynchronous serial communication interface
(SCI) between the front end and the rest of the monitor, via the isolation block. The
transmission format has a start bit, nine data bits, no parity, one stop bit, and a baud rate of
115.2K. The nine bit format is somewhat unique to Motorola products. It consists of eight
normal data bits, plus one extra bit sometimes referred to as the “address mark” bit. It was
originally intended to designate addressing information in multidrop configurations. In this
monitor, it is used as a means of marking the end of a packet. Each character thus contains a
total of 11 bits, and is transmitted in nominally 96 s. Packets sent from the microcontroller to
the CPU have a variable number of bytes, and consist of several data items followed by a
checksum and terminator. Each data item is internally identified as to its contents. Packets
received by the microcontroller always consist of two bytes, a single command byte
followed by a confirmation.