Measurement Computing Serial488A - Manual (Page 71)

6.1
Service Information
6.1 Factory Service
If problems are encountered in using the Serial488A you should first telephone
the factory. Many problems can be resolved by discussing the problems 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
The Heart of the Serial488A is a 6809 microprocessor [U101] supported by 8K
bytes of firmware EPROM [U102 (2764)] and 32K bytes of static RAM [U103
(58256)]. A Versatile Interface Adapter [U104 (65B22)] 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)]. The IEEE 488 bus interface is accomplished by a TMS9914A [U106]
controller with drivers U107 and U108. The serial interface is provide by the UART
[6551 (U105)]. If RS-232 levels are chosen, they are provided by the RS-232
transceiver (U209). If RS-422 levels are selected, the differential driver [26LS30
(U207)] and receiver [26LS33 (U208)] are used.
The internal DIP switches [SW1, SW2 and SW3] are read via 74HCT244 tri-state
buffers [U201, U202 and U203]. Power is supplied by an external unregulated 9 volt
wall mount supply. Regulation to the required +5 volts is provided by U206 [7805].