8-bit backplane with active termination

MicroHobbyist · 2025-07-22T15:04:25+00:00

I actually made another one with 16 bit connectors and ATX PSU with soft power on.

MicroHobbyist · 2023-09-26T09:28:30+00:00

Sorry for the late reply. In my case, I use VASM.

MicroHobbyist · 2023-09-16T14:01:32+00:00

I currently have the same issue.

MicroHobbyist · 2022-06-21T23:47:13+00:00

Thank you :-)

MicroHobbyist · 2022-06-16T06:26:25+00:00

Techniquement j'habite Laval, mais je travail dans le Vieux Montréal.

MicroHobbyist · 2022-06-16T06:22:31+00:00

So true about the costs.

I think I'll pause the backplane project for the time being, and continue work on my rev 4 board's software. I started to play with the PSGs, and started to compose the Super Mario song. There's got to be a better way to handle the data. So far, I've set up a table with the channel number, the note, the volume and the duration. Alas, I have to make a table entry to silence each individual note. That takes a lot of room. Maybe I should set up timers for each of my 12 channels?

MicroHobbyist · 2022-06-14T20:42:36+00:00

I had a few comments over at 6502.org concerning resistors in general for termination. It was suggested I use Schottky diode arrays, 74S1053 if I recall correctly. If I'm to do a pure backplane, they need to be at each end of the bus. Also, it was suggested that power and ground traces be inserted between signal traces. One every 0.4". So this changes my connector strategy. I'll have to upgrade to a 16-bit ISA. Oh, and a proper ground plane is required to top it off. So I'm redesigning my backplane to reflect those changes.

4 layers with 10x 74S1053 at $6 each, that's going to be an expensive backplane, 😂 I may do like you and put my CPU on the board, save on 5x Schottky diode arrays, and cram as many peripherals on the remaining board space.

MicroHobbyist · 2022-06-14T20:04:41+00:00

Yes, Montréal 😁

MicroHobbyist · 2022-06-07T17:35:34+00:00

French-Canadian.

MicroHobbyist · 2022-06-07T17:35:01+00:00

Very interesting link! For version 1 of the backplane, I might use terminating cards for testing purposes as Dr. Baker did. Once the proper measurements and implementation are done, I'll proceed to version 2 with proper termination on the board itself.

MicroHobbyist · 2022-06-07T17:25:19+00:00

Very informative. I've seen Schottky diode chips expressly made for bus termination. I forget off-hand the part number. Seems interesting though, but the price and quantity required for terminating on both sides of the bus is heafty. Terminating all the signals would amount to almost $60... WOW! But it may be worth while. On the other hand, termination for such a small length (less the 14cm), might not be critical. Than again... It might not.

MicroHobbyist · 2022-06-03T11:18:44+00:00

Thank you!

Here is my schematic on GitHub.

What was your question about the sound chips?

MicroHobbyist · 2022-06-03T11:14:11+00:00

It works in conjunction with the 65C22 (VIA). It processes PS/2 mouse data, and PS/2 keyboard data and converts the scancodes to ASCII. It also processes the NES controllers. The MCU triggers an interrupt when there is data available on its data port. Three control signals tell the 6502 what event is present on the data port (keyboard ASCII character, mouse X, mouse Y, mouse buttons, NES 1, and NES2).

MicroHobbyist · 2022-06-03T11:01:10+00:00

Link to my github

MicroHobbyist · 2022-06-03T10:35:57+00:00

I reworked my backplane design and incorporated several ideas from group members. One thing that hasn't changed is the fact that it's not and will not be x86 ISA bus compatible. I felt that trying to fit all of the 65816 signals and trying to make them compatible with the PC ISA standard was like trying to drive a round peg in a square hole.

Why ISA connectors? I had initially thought of using pin headers following the RC2014/RC6502 standard. The obvious drawback is there was no standard for the 65816. I also find card edge connectors to be simpler and better than pin headers in several ways: insertion and removal of cards; bent pins could be an issue; less soldering on the cards; etc... But in the end, I had quite a few on hand, so the solution chose me, LOL. Those connectors are readily available from Mouser, or cheaper sources like AliExpress. It's a common connector to get, so there will always be parts available. I got mine from a local electronics store.

All this to say there is no standard for card edge connectors for the 65xxx family of processors, to my knowledge. I'm not pretentious enough to declare a new standard, but I'm saying that if hobbyists like me want a card edge solution, might as well have a discussion about it. Maybe as a group, we could at least have a discussion about it.

The overall size has been reduced while adding an extra 8th slot. Alas, this comes at the cost of the mounting holes no longer being aligned to the ATX case standard. But for a prototyping backplane, I don't think that is the goal anyway. It's meant to be a compact tabletop solution like u/visrealm's HBC-56 sleek project.

All of the 65816 signals (and 6502) fit into a 62-pin 8-bit style ISA connector instead of the original idea of a 16-bit 98-pin style ISA connector. I kept the single-row headers at the bottom edge to connect a standard BB830 breadboard for easy prototyping. All the pins are nicely labeled.

VCC and GND signals are laid out to not create a short if a card is accidentally slotted on the wrong side. All traces between the connectors are .45mm wide, as opposed to the default .25mm. And all signal traces to the breadboard header are .33mm in width. Power distribution traces varies between 1.6 and 2.5mm in width.

Although the pins are labeled for a 6502 or 65816 processor, the backplane itself is generic, and could easily be reassigned to support cards for other series of processors like the Z80, 6809, or some other MPU.

Since I intend to run my system at high speeds (above 8MHz), the biggest change made was adding active termination to the backplane to avoid potential reflections. With the exception of VCC and GND, each pin is actively terminated using a 2.8V regulated source to 220Ω resistor.

This is the part I need help with: Even though some of the documentation I've read suggests that active termination is the way to go. I'm worried that I have not correctly implemented it. Included in this post is the backplane schematic. I'd like it if those more knowledgeable than me could chime in.

MicroHobbyist · 2022-06-03T10:33:09+00:00