Measurement Computing Extender488 - Manual (Page 33)

Section 4 Service Information
4.1
SERVICE INFORMATION
4.1 Factory Service
Measurement Computing maintains a factory service center in Norton, Massachusetts. If problems
are encountered in using the Extender488, you should first telephone the factory. Many problems can be resolved
by discussing the problem with our applications department. If the problem cannot be solved by this method,
you will be instructed as to the proper return procedure.
4.2 Theory of Operation
At the heart of t he Extender488 is a 6809 microprocessor [U101] supported by 8K bytes of
firmware EPROM [U102 (2764)] and 8K bytes of static RAM [U103 (6264)]. A counter [U105 (65C22)]
contained in the VIA is used to generate real time interrupts for the firmware operating system. The front
panel annunciators are driven by the VIA through an inverter/driver [U113 (74LS04)].
Serial communication to the mating Extender488 is accomplished by a UART [U105] thru the RS-
422 receiver [U112] and driver [U111]. For the Extender488/F it is accomplished by the UART and fiber
optic transmitter [U202] and receiver [U203].
The IEEE 488 bus is monitored by a 9914A IEEE bus device [U106]. Circuitry surrounding the
9914A (U115 and U116) enables the device to monitor all bus activity, and to initiate bus activity when
required.
Decoding of the microprocessor address space is accomplished with a pr ogrammed PAL [U11 0
(16L8)]. Below is the memory space allocation.
Address Device Part Number Function
$6000-$7FFF U103 6264 Static RAM
$A000-$A007 U106 9914A IEEE Controller
$A800-$A803 U105 6551 UART
$B000-$B00F U104 65C22 VIA
$E000-$FFFF U102 2764 Programmed EPROM