Video Repeater

Using, learning, programming and modding the Gigatron and anything related.
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axelb
Posts: 16
Joined: 07 Jan 2021, 06:27

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 1191 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 1219 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 1219 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.


https://github.com/axelb68/Gigatron-Video-Repeater
Hans61
Posts: 40
Joined: 29 Dec 2020, 16:15
Location: Saxonia
Contact:

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:
FIFOv3.zip
(315.72 KiB) Downloaded 64 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
20210527-171006-037-fifo2-web.jpg (72.94 KiB) Viewed 1163 times
walter
Site Admin
Posts: 126
Joined: 13 May 2018, 08:00

Re: Video Repeater

Post by walter »

Got my video repeater in the mail today, thanks Axel!
lb3361
Posts: 82
Joined: 17 Feb 2021, 23:07

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.
jtompuri
Posts: 4
Joined: 08 Aug 2020, 21:25

Re: Video Repeater

Post by jtompuri »

I am planning to build the video repeater. However, it seems quite hard to find the FIFO chip. Any suggestions where to look for it in Europe? Fast 15ns chips like IDT7201LA15TP seem to be quite expensive (20-30 €). You can get IDT7201LA25TP for 3-5 € from China but the shipping takes forever.
Hans61
Posts: 40
Joined: 29 Dec 2020, 16:15
Location: Saxonia
Contact:

Re: Video Repeater

Post by Hans61 »

At Aliexpress I have IDT7204L15TP for 1.50€, the wait usually does not bother me.
The video repeater I also wanted to try as soon as possible.
At eBay I got the IDT7202LA25TP for 6,99€ (3 days delivery)
https://www.ebay.de/itm/123904948478
walter
Site Admin
Posts: 126
Joined: 13 May 2018, 08:00

Re: Video Repeater

Post by walter »

I have ordered 5 PCBs. I have bought enough components for the 5, will keep one to myself, the others I can provide for a small fee (I am not planning on making money on them). Latest stuff will come in the coming week I hope.

Aliexpress has just become more expensive now Europe has agreed to charge VAT on all shipments, also those < €22. Here, you pay 21% VAT and €4 handling per shipment.
jtompuri
Posts: 4
Joined: 08 Aug 2020, 21:25

Re: Video Repeater

Post by jtompuri »

Thanks for the tips! I didn’t check out AliExpress. I finally found a pair of QS7204-15 from Germany for total of €15 icluding shipping. In Finland we pay 24% VAT and customs handling fee (2,90€) for the Finnish post office, so ordering cheap stuff from UK and China doesn’t make sense any more.
walter
Site Admin
Posts: 126
Joined: 13 May 2018, 08:00

Re: Video Repeater

Post by walter »

If you're interested, send me a PM. I think I'll sell them for €10 excluding shipping. That includes the PCB (green), all the ICs (I use the AM7205A), a socket for the AM7205A, caps, jumper, header pins. You'll have to solder it yourself. Shipping to Europe will be around €5 I think.

And here's my Gigatron with the video repeater and the RAM+I/O expansion.
repeater and RAM+I/O expansion
repeater and RAM+I/O expansion
repeater.jpg (460.1 KiB) Viewed 918 times
walter
Site Admin
Posts: 126
Joined: 13 May 2018, 08:00

Re: Video Repeater

Post by walter »

I have the parts for the 4 excess PCB's I ordered.

So, for sale are 3 kits (one already gone) of axelb's video repeater design, which includes:
- green PCB as created by Hans61
- pins that go into the socket that contains the 74HCT377
- jumper and pins
- 2x 74S113
- 1x 74LS00
- 1x 74HCT574
- 1x Am7205A-15
- 1x socket for the Am7205A-15
- 6x capacitor

Note I do not include sockets for the other ICs, nor an 74HCT377 as you can use the one from your main board. Might you need an additional 74HCT377 you can ask for it.

Edit: they're all sold.
IMG_20210702_174803.jpg
IMG_20210702_174803.jpg (4.04 MiB) Viewed 826 times
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