Page 2.4
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 2
Functional Overview
MK 14 - 10 Main Board
Power Supply
The mains supply is full wave rectified to produce an unregulated dc supply of approximately 30
V (+U). This voltage is used by a switched voltage generator to pUD (13.5V). +UD
charges the battery when mains is connected. When mains is not connected, +UD is the battery
voltage.
An ON/OFF control logic swUD to three voltage regulators. The unit is switched on
directly from the keyboard and then held on from the CPU . Detection of overvoltage on either
the 5.2 V or 24 V supplies directly switches the unit off. Similarly when an undervoltage is
detected on +US (indicating overcurrent) the unit is directly switched off.
The mains LED is lit directly when mains is connected. The same circuit also monitors the switched
dc supply (+US) and activates BATT when the unit is switched on and mains is not
connected (i.e. the unit is running on battery power).
A Battery low signal (BLOW) is set to logic 0 when battery voltage (+US) falls to 11.3 V. A
circuit compensates for voltage drop when the printer stepper motor is active and the BLOW
signal is active only at 10.3 V.
Note :
The battery voltage is also monitored directly by the CPU which switches the unit
off when the voltage falls below approximately 9.4 V.
Program and ECG Memory
A FLASH EPROM (electrically erasable) with a capacity of 4 MByte contains the unit software
(512 kByte) and the stored ECG data. The two memory blocks can be independently erased. It is
possible to update the software via the RS-232 serial interface. This can only be done by a service
engineer. Procedure outlined in Chapter 1.
Serial EEPROM
The serial EEPROM (U48) stores the unit base settings.
Thermal Print Head Controller
The Thermal Print Head is controlled by a print head controller and a CPU timer circuit. The print
head controller serialises the data for the print head and the timer circuit controls how long current
is applied to the head, and thus the intensity of the printout.
Printer Timing
Strobe generation is controlled by the CPU when one complete pixel line of data is ready to be
written. Pulse length of STRB1 and STRB2 (each of which controls half of the pixel array)
depends from TPH temperature and so form the pulse width of the TPHT signal.
Note:
TPH temperature reading is described in Chapter 5.
Paper Mark
The pulsed paper mark signal from the printer is fed to a comparator. A detected papermark
supresses any (logic 0) pulses of PMARK at the output of comparator U42.
