Mindray Passport 5-Lead, Service Manual

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Block Diagrams Theory of Operation
2 - 12 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
The buffered active lead signals are applied to a conventional Wilson network, consisting of
resistors R11, R13, R16, R17, R21, R23, R24, R27 and R29. These resistors provide the
weighted leadwire signals when displaying leads aVR, aVL and aVF, as well as the “Wilson
Central Terminal” (WCT) for display of lead C. For example, lead aVR is defined as 50%
each of the LL and LA signals taken as one ECG amplifier input, and RA as the other.
Resistors R11 and R17 perform the 50% weighting of signals LL and LA. Similarly, the C lead
is defined as the voltage on the C leadwire, measured against the average of the RA, LA and
LL voltages, known as the WCT. Resistors R13, R21 and R27 develop the WCT voltage. The
correct points for each lead display are picked off of the Wilson network by multiplexers U2
and U3. The outputs of these multiplexers are called ECG+ and ECG-, corresponding to the
positive and negative inputs of the ECG amplifier on the front end board. They are carried to
the front end board on pins 4 and 10 of J1, which mates with the connector installed in the
former site of U3 of the front end board. On the front end board, jumpers installed in position
U4 carry these signals to lines F and G, which are the inputs of ECG differential amplifier
U33.
The lead select multiplexers, U2 and U3, as well as the drive multiplexer, U4, are controlled
by three select lines, which select one of 7 leads or the ECG calibrate function. The lines
LSEL0 and LSEL1 correspond to the original LS0 and LS1 lines used in the 3-lead version. An
additional line, LSEL2, has been added to allow selection of the added leads. The network
R14-C11 in this line provides isolation between the microcontroller and the ECG
multiplexers, and is identical to the networks found in the LS0 and LS1 lines on the front end
board.
Lead fault detection is performed by measuring the DC voltage of the ECG+ and ECG-
signals. Resistors R33 and R34, together with diodes CR5 and CR6, select the lower (more
negative) of the two voltages on these points, and routes it to signal A, which is passed to the
lead fault comparator on the front end board. The operation of the lead fault detection takes
into account simulators which may short the ECG and pressure circuits together. The Passport
will not show a spurious lead fault with most such simulators, although it may fail to detect
certain lead faults when the pressure is connected to the simulator. Since this is a different
operating mode than the original Passport lead fault detection, the lead fault comparator has
been modified, as is discussed below.
Front End Board
The ECG section of the front end board has been modified from the previous 3-lead system.
The ESIS filters and lead bias resistors have been removed entirely. U3 has been replaced by
a connector which mates with the daughter board. Multiplexer U4 has been replaced by two
jumper wires, which allow the ECG signal from the daughter board to pass to the ECG
amplifier U33, as has been discussed previously. The additional lead selection line has been
added by connecting pin 27 of U38 to pin 1 of the connector used in place of U3. This line
is activated when enhanced microcontroller and Datasette software is used. In order to allow
the software to be backwards compatible with the 3-lead system, the 5-lead board is
identified by grounding pin 21 of the microcontroller, which is high on the 3-lead systems.