Measurement Computing Parallel488 - Manual (Page 45)

Section 6 Service Information
6.1
Service Information
6.1 Factory Service
IOtech maintains a factory service center in Cleveland, Ohio. If problems are
encountered in using the Parallel488, you should first telephone the factory. Many
problems can be resolved through discussions with our applications department. If the
problem cannot be solved by this method, you will be instructed as to the proper return
procedure.
6.2 Theory of Operation
At the heart of the Parallel488 is a 6809 microprocessor [U201] supported by 8K
bytes of firmware EPROM [U102 (2764)] and 32K bytes of Static RAM [U103 (84256)].
A Versatile Interface Adapter [U104 (R65C22)] is used to generate real time interrupts
for the firmware operating system. The front panel annunciators are also driven by U104
through an inverter [U113 (74LS04)].
Handshake lines for the parallel port are controlled by a programmable interface
adapter ("PIA") [U202 (6821)]. In the IEEE to parallel mode, this same PIA is used to
output 8 bits of data to the parallel port. In the parallel to IEEE mode, an 8 bit latch
[U206 (74LS373)] reads data from the parallel port.
Decoding of the microprocessor address space is accomplished with a
Programmable Logic Array [U110 (16L8)]. Below is the memory space allocation.
Address Device Part Number Function
$2000-$7FFF U103 84256 Static RAM
$8000 U206 74LS373 Parallel data receiver
$9200 U202 6821 Parallel handshake/data out
$A000 U106 9914A IEEE Controller
$B000 U104 R65C22 LED Driver
$E000-$FFFF U102 2764 Programmed EPROM