Service manual

PT300, Service Manual

Rev B, October, 2000

Now for the messy details of implementation; U17 is driven through the

integrate, deintegrate, and auto-zero cycles by a counter U12. This counter is

loaded by the micro processor to start the cycle, and U12 pin 6 output is used as

an interrupt to signal that the conversion is over. The counter's clock is driven

by ones output from the Counter-Timer-Chip, U13. The deintegrate phase is

signaled by ones on the control lines A and B into U17; this is combined with the

comparator output from U17 using Boolean AND function; gates clock pulses

into another section of U13. When the Micro is signaled that a conversion is

finished it reads a 16 bit number in U13 that is proportional to the weight on the

load cells. The micro clears the counters U12 and U13 pin 14, starting another

conversion cycle. Note that the MIN/MAX bit out of counter U12 is used as the

auto-off signal. To turn the scale off the computer does not service the end of

conversion interrupt. This also acts as a dandy watchdog timer, if the micro is

incapacitated it does not service the interrupt and the scale shuts off.

Microprocessor section:

This is the logic and timing portion of the system. The computer portion, user

interface and input/output sections fall in this category.

CPU:

U2 is a standard CMOS Z80 processor chip. Y1, U3A, U3B, and

associated parts form a 1.2 mHz clock. R37 and C20 form the power on

reset pulse.

Memory:

U5 is a 2K x 8 bit static RAM memory. A 27C64 EPROM (U1) holds

program and setup information. There is no NOVRAM or related

parameter storage. All setup parameters are set at fabrication and are

held in the EPROM device.

Device Selector:

U9 and several low level gates select all devices as memory. No refresh

or I/O cycles are needed or used.

Switches & Reader:

U4 reads all the switches except ON and OFF. Pin 17 of U4 reads the

serial input line, pin 4 reads the low battery line.