transferred to the processor where the measurements are calculated
and presented in the display.
The CPU comprises a Motorola 68HC11A1 operating at 8 MHz,
which gives a BUS frequency of 2 MHz. The UART in µP is used on-
ly during boot up. The pulse accumulator in µP is used to record mea-
surement data.
A Maxim µP supervisory circuit, with a 4.65 Volt reset, is also used
to monitor the 12 Volt power, and to ensure battery power to the
RAM when the unit is switched off. U1080 and DS2404 are timer cir-
cuits. These provide the system with real time. U 1160 is an address
decoder. UI120 is an address latch for the multiplex address bus. µP
has a 64 KB address range. UI130, together with U1150, is used as a
bank switch to address the process circuit's RAM and flash ROM.
Refer to Schematic Diagram 4 (Memory QA-90). The memory com-
ponent comprises 128 KB RAM and 512KB flash ROM for storing
programs. The CPU module is programmable, and may he repro-
grammed with new software supplied by Metron. All transistors in the
memory circuit are used to supply the flash ROM with a 12 volt pro-
gramming voltage.
Refer to Schematic Diagram 5 (Printer and Display IF QA-90). The
printer and display interface comprises PD71055, PIO that is used to
write to display and to the Centronics output. Data to Centronics goes
via LS05, which has an open collector output. A strobe pulse to the
Centronics port is established through U1410A-D.
Keypad scanning and the RS-232 interface are established by UI300,
DUART. The keypad is a standard keypad matrix.
U13 I0, MAX238 is a RS-232 transmitter/receiver between DUART
and the RS-232 port.
6.4 Component Parts
COMPONENT PART DESCRIPTION QTY.
30-4412400 DS2404S SMD 1
30-4474573
74HCT573T SMD 1
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