Video Repeater

Using, learning, programming and modding the Gigatron and anything related.
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Video Repeater

Post by axelb »

This is an updated thread for the Gigatron Video Repeater which was already presented in "Possible ways of speeding up the Gigatron".

The video repeater is a small add-on board to offload the task of duplicating video lines. Every first line out of four is written to an on-board memory and autonomously transmitted 4 times to the VGA output. By this the Gigatron can always be run in mode 3 and is still producing a screen output without blanked lines.

Main component is the asynchronous FIFO memory chip SN74ACT7201 (or compatible, see below) which is constructed with dual-port SRAM and internal write and read address counters. The read pointer can be reset independently of the write pointer for retransmitting previously read data.

Working principle of the asynchronous FIFO memory chip
Working principle of the asynchronous FIFO memory chip
Asynchronous FIFO.png (57.71 KiB) Viewed 173 times

The first video line (out of four) writes each pixel to the memory which is followed by an immediate read. This is like a flow-through operation
which simply copies the pixel information back to the output.
A retransmit of the complete line is then performed for the next three HSYNC cycles.

Video Repeater schematics
Video Repeater schematics
gtfifo4.png (121.62 KiB) Viewed 201 times

The schematics of the repeater shows connector U1A which can be plugged into the U37 output register socket of the Gigatron. HSYNC is divided by two JK flipflops (U2). The result is used to initialize the internal read and write pointers of the SN74ACT7201 to the first location. A 74HCT574 latches the memory output and passes it back to connector U1A for VGA output.
By means of a "scanlines on/off" jumper (JP1) the primary line can be blanked-out in order to create a "scanline look" similar to mode 1 of the Gigatron.

Assembled repeater
Assembled repeater
VideoRepeater.JPG (174.63 KiB) Viewed 201 times

So far I tested the board with different clones/extensions of the SN74ACT7201. IDT7201-15, IDT7202-25, and QS7204-15 worked fine.
The picture shows a mixture of 74F, 74ACT, and 74LS components which is not important, however. It is sufficient to use the HCT versions or even LS for the flipflops and the logic.
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Re: Video Repeater

Post by Hans61 »

Here again is my version of the PCB from Axelb's repeater. I made it mechanically to fit next to the 128K expansion from lb3361. It works very well and gives us 3 of 4 video lines to calculate with. Here are the Eagle and Gerber files:
(315.72 KiB) Downloaded 13 times
It's just been a quick fix with the auto-router. Mechanically it is a bit of a shaky solution due to the geometry. A better solution would be to combine the repeater with the expansion on one PCB. In this case it would be nice to be able to control the switching between 3 and 4 video line mode via software. Maybe with a MCP23S17 or other. In that case a reset state wouldn't matter either, then it just happens to be the 3 or 4 video line mode at the beginning. I'm not sure now, but doesn't the 128K expansion also need a defined state of the RAM banks and isn't this established via the ROM when booting?
20210527-171006-037-fifo2-web.jpg (72.94 KiB) Viewed 145 times
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Re: Video Repeater

Post by walter »

Got my video repeater in the mail today, thanks Axel!
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Re: Video Repeater

Post by lb3361 »

Hans61 wrote: 05 Jun 2021, 17:44 I'm not sure now, but doesn't the 128K expansion also need a defined state of the RAM banks and isn't this established via the ROM when booting?
One of the very first thing the Rom does is instruction ctrl(0x7c) which precisely resets the expansion into a safe state.
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