Mindray Passport 5-Lead, Service Manual

Page: 238 / 544

Block Diagrams Theory of Operation
2 - 36 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
When serial data is received from the Oximeter Modules, the microcontroller buffers the data
until a complete packet has been received. The packet is then validated by means of a
checksum. If valid, the microcontroller makes the first byte available to the bus interface
logic, and interrupts the Passport CPU. The Passport responds by reading the first byte. A
flag indicating the availability of data (first and subsequent bytes) is provided in the status
register. By polling this flag bit in a tight loop, the CPU is able to read the subsequent bytes.
The CPU is able to identify the packet’s byte count based on the first byte read, and therefore
knows how many additional bytes to read by polling.
A similar procedure is used to transmit serial data. The CPU polls a busy flag in the status
register. When this flag is clear, one byte of data is written to the PM/NIM. The busy flag
becomes immediately set. As soon as the microcontroller accepts and buffers this data, the
busy flag is cleared by the microcontroller, so that the CPU can write the next byte. This
process continues until an entire packet has been written. When an entire packet has been
written, the PM/NIM validates the packet by means of the checksum, and proceeds to
serially transmit it to the SpO2 Module.
It is possible to assert or remove reset to the microcontroller by writing to the control register.
Whenever the ISA bus signal RESETDRV is active, the microcontroller reset will be asserted,
and will remain asserted after RESETDRV is no longer active. Internal power-on-reset logic in
the EPLD asserts reset in a similar way. . As the microcontroller passes from the reset to the
active state, it activates the SpO2 Module reset line and enters a data echo mode. Only
after receiving a command from the host will it exit the echo mode and release the SpO2
Module reset. This insures a coordinated start-up of the PM/NIM and SpO2 Module.
Read-Write Logic
The figure below shows the portion of the EPLD, U4, which provides the read and write logic.
The four most significant ISA address lines, SA16..SA19, are decoded. Two ports are
needed, one for control and one for data. The selected port addresses are at ISA 80000
hex for data and ISA 90000 hex for control/status. These correspond to 900000 hex and
920000 hex, respectively, in the Passport CPU address space. The address assignments are
unique to allow the Passport to easily determine on bootup which modules are connected to
the ISA bus. The Passport software accomplishes this by attempting to address the module.
If it is not present, a bus exception occurs indicating the absence of a working PM/NIM.
When an OEM SpO2 module is used in the Passport a Software command is used to
determine whether a Masimo SpO2 Module or Nellcor SpO2 Module is present.
When a write is made to the status port address, the inverted state of D0 is latched in flip-flop
F/F 3 inside the EPLD, U4 (Figure 2-14 on page 2-35).