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The VDB-A was a video terminal interface for S-100 based computer systems. It provided a totally programmable C.R.T. display, consisting of an 80 character by 24 line display, blinking cursor, text highlighting, and text formatting, etc. The Intel 8275 was fully implemented, including the light pen feature. (via software linkages in the program EPROM). A complete parallel-input keyboard port was provided, and a portion of the on-board 2114 static RAM was used to allow the user to type commands ahead to the host computer while it was executing other code. The on-board Z80 CPU and 2K program EPROM provided lightning fast keyboard and video operations.
A second 2716 EPROM provided the full ASCII character set with lower case descenders, while an optional third EPROM could be used for an alternate character set or custom graphics. Pre-programmed EPROMS could be purchased separately. The on-board RAM (3K) was used for the keyboard buffer, video and attribute data, and as a scratch pad/stack for the CPU.
The board could be configured as either I/O port, or memory mapped, and could be changed from one to the other under keyboard or software control. In the I/O mode, the on-board RAM is not addressable by the host CPU (i.e.: did not occupy system memory map), and all video and screen attribute data flowed through the I/O ports. In the memory mapped mode, the I/O port were still fully active but the screen portion of the RAM could be written to by either the host CPU, or through the I/O port, although the video data RAM could not be directly read by the host CPU.
The manual and schematic for this board can be obtained here.
This page was last modified on 10/25/2013