Smiths Medical Medfusion 3000 Series, Technical & Service Manual

Page: 56 / 184

Theory of Operation
50
40-5648-51B
External DC power conditioning /
detection description
The external DC power input section limits current
through a resettable fuse, both common and differ -
ential mode noise suppression and transient voltage
protection. The
resettable fuse
limits current into the
pump from an external DC source. Noise filtering
is performed by input and output capacitors on the
conditioning circuit together with a common mode
choke.
The external power detector uses a
zener diode to pro-
vide a valid logic high level when external DC power
is applied. The active voltage for extrnl_pwr_de -
tect is nominally 5 volts . The threshold detector U3
provides immediate detection, through the signal
battery_swx_on*, of the external DC source being
removed.
• A battery switch toggles to internal battery if both
AC and external DC power are removed.
• The threshold detector also prevents the battery
charger from charging the internal battery until
the external DC power exceeds 5 volts.
Infrared serial data port description
The infrared serial port interfaces directly with the
main microprocessor’s asynchronous serial commu-
nication pins. The infrared port supports short trans -
mission distances of approximately 3” or less and a
maximum baud rate of 9600.
The Infrared Receiver signal from U1 is conditioned
by the comparator circuit of U4 to generate a valid
logic state to the microprocessor.
The
Infrared Transmitter signal ir_xmt_data is at a
logic high during its inactive state. A logic low on ir_
xmt_data enables the transmitting IR diode through
the transistor Q3.