HP 300

Video Circuits
All the electronics for sending video to a nlonochromatic bit-mapped display are also part of the
985~31-66511 board .. The video circuits have the following functions:
• Frame Buffer, consisting of eight 16K by 4 RAM chips for 400 lines of 1024 pixels in each
line.
• Frame Buffer Controller, the same as on the Video Output Boards.
• Display ID ROM, in an 8 K-byte ROM
•
A video shift-register.
• A pixel clock.
When the computer application uses a separate video board, a switch on the processor board
must be set to the 1 position to disable the logic for the processor's on-board video circuits.
The frame buffer for these video circuits is memory mapped to addresses in the display frame
buffer block
of RA~1. A single bit-mapped plane organized as one byte per pixel is used. Due
to the resolution of 1024 horizontal by 400 vertical dots, the pixels are not square. When the
fraIne buffer is read, invalid data is returned for the unused planes.
Keyboard Electronics
Features on this processor board is in the keyboard interface electronics, and a battery backed-
up real-time clock. Software gains access to this real-time clock through the 8042 keyboard
controller.
Boot ROMs and Self-Test LEDs
98561-66511 has a Boot ROM whose instructions are executed by the CPU shortly after power
up.
Test LEDs are on
the processor board to help service personnel in trouble shooting. The Boot
ROM functions are explained in Chapter 4.
Other Circuits
An Intel 8042 Universal Peripheral Interface 8- Bit Microcomputer performs several functions on
the processor board:
• Interfaced to a SN76494 digital complex sound generator IC.
•
Provides several 10 millisecond resolution timers.
• Performs key-code conversions for the keyboard.
• Manages the HP-HIL Interface.
• Has a battery backed-up Real Time Clock tied to its peripheral data path.
Functional Description 47
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com