| Project CheapBus [message #5555] |
Mon, 19 November 2018 07:56  |
ab0tj
Messages: 70
Registered: October 2015
Location: Colorado
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For a while now I've been eyeing the RC2014 system, mainly because I thought it would be nice to be able to create board designs that take advantage of the $2 PCB specials out of China. Every time I look at the bus specs I'm a little bit underwhelmed, though. So lately I have been working on my own bus design. I call it "CheapBus" because the goal was to be able to get prototype boards for about $5 (excluding parts of course, so PCB+connector). Here's the "feature list":
- Basically a bastardized version of the ISA bus, allowing the user to take advantage of a lot peripheral chips designed for the PC compatibles
- Standard board size is 100x100mm, the usual limit for the "$2 special" from Chinese board houses. I have been using surface mount devices quite a bit lately so this should be plenty of room
- Uses a 120-pin connector that is just over $3 in single quantities. I debated on using a PCI card-edge style connector, but this would actually increase costs as the edge connector should be gold plated.
- Includes all the standard ATX voltage rails, 3.3V being nice to have for a lot of newer devices that I might want to interface to the system
- Includes a SMBus-style I2C bus. The backplane I've been working on has an ATTiny microcontroller on this bus that can monitor system voltages, control system power, etc.
- 16 data lines and 24 address lines (fully latched), can address up to 16MB.
- Has a line similar to the S100 phantom line, allowing memory mapped IO to easily overlap system memory.
- Supports controller-driven or bus mastering DMA
- Active termination at both ends of the bus
Why not just use ECB, you might ask? That's a fair question, but it doesn't have some things I'd like to have for future projects I have in mind. It won't hurt my feelings if nobody else wants to use it  I have been working on the backplane first, attached is what I have so far. I may need to redesign the termination circuit... I copied it from the RetroBrew S100 backplane and the LM4250 op-amp seems hard to come by these days.
Once I finally get them manufactured, I'll have a couple extra backplane and breakout boards if anyone wants to join in the "beta test".
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| Re: Project CheapBus [message #5756 is a reply to message #5555] |
Mon, 17 December 2018 07:39  |
ab0tj
Messages: 70
Registered: October 2015
Location: Colorado
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So at this point I have a functional backplane and breakout board 
Maybe someone that is more fluent in analog electronics can help me out, though. The termination circuit was copied from a classic design as seen here and here, verbatim other than using two 560-ohm resistors (one at each end of the bus) instead of 270 ohm resistors at one end of the bus. Attached is my implementation of the circuit. Powered from 12V, I can get a stable 2.7 volts out of it until I short more than two data lines to ground or pull more than two up to 5V, when it starts to go out of regulation. Seems like this circuit should be able to source or sink a lot more current than that. Am I doing something wrong?
[Updated on: Mon, 17 December 2018 07:46] Report message to a moderator |
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| Re: Project CheapBus [message #5760 is a reply to message #5555] |
Mon, 17 December 2018 20:15 |
ab0tj
Messages: 70
Registered: October 2015
Location: Colorado
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Ok, I figured it out. I used the wrong footprint for the TIP29 and TIP30 transistors so they were not contributing to sourcing or sinking current. After cutting the leads and rearranging them it seems to be working as expected.
EDIT: Looks like this is a problem on the 9-slot S100 backplane as well, the TIP29 and TIP30 are BCE, not EBC
[Updated on: Tue, 18 December 2018 20:55] Report message to a moderator |
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| Re: Project CheapBus [message #5815 is a reply to message #5555] |
Thu, 03 January 2019 14:00 |
ab0tj
Messages: 70
Registered: October 2015
Location: Colorado
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Continuing down this path of doing whatever the heck I want even if it isn't especially practical, I placed an order for 5 boards today:
- 8085 CPU board with an MMU allowing it to access the entire 24 bit memory space
- DRAM board, supporting a 16MB 72-pin SODIMM module
- Ethernet board with the same WIZ830MJ module I've been using on the S100 bus
- A Raspberry Pi based board that will handle console, disk, and loading the memory contents at boot (ROM and more retro-friendly disk and console can come later)
- A board that supplies a 8259 interrupt controller, 8254 timer, and DS12885 real-time clock
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Attachment: 8085Board.png
(Size: 107.10KB, Downloaded 615 times)
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Attachment: DRAMBoard.png
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Attachment: Ethernet.png
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Attachment: PiBoard.png
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Attachment: PPRBoard.png
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| Re: Project CheapBus [message #6748 is a reply to message #5555] |
Sun, 10 November 2019 14:57 |
ab0tj
Messages: 70
Registered: October 2015
Location: Colorado
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Since CP/M has been working fine, I'm now bringing this thing into the 16 bit era! End goal is MS-DOS compatibility with EMS memory support.
I somehow used the wrong footprint for the bus connector so it ended up on the back side of the board. I guess that means I'll have to respin this one at least once, but so far it appears to be working. Now time to write some software to support it...
[Updated on: Sun, 10 November 2019 14:59] Report message to a moderator |
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| Re: Project CheapBus [message #7333 is a reply to message #6838] |
Fri, 27 March 2020 10:41 |
ab0tj
Messages: 70
Registered: October 2015
Location: Colorado
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Getting back to this project now that I'm stuck at home... After a simple fix to the GAL code on the 186 board it is now running code! It's funny how exciting it can be to just blink a LED. "Hello world" comes next but maybe MS-DOS as a long term goal?
[Updated on: Fri, 27 March 2020 15:59] Report message to a moderator |
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| Re: Project CheapBus [message #7861 is a reply to message #7860] |
Thu, 09 July 2020 17:03 |
djmartins
Messages: 40
Registered: February 2018
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I had to add this is a pretty cool setup you are doing here.
Love to see you put info on the wiki so others could possibly design for this bus.
You had me at the word "cheap" but it looks like it can make a cool system!
Like b1ackma1ler said: "That's an amazing amount of developement for 1 person"
A mini DOS box with EMS memory.....
[Updated on: Thu, 09 July 2020 17:04] Report message to a moderator |
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| Re: Project CheapBus [message #7871 is a reply to message #7865] |
Tue, 14 July 2020 11:14 |
bigpete
Messages: 7
Registered: May 2020
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Junior Member |
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This is really cool. I see in the Github you have a 286 board. EDIT: I opened it up and see it's just a placeholder file. Are you going to try to get DOS working on the 186 first? Were you able to get that working yet or is it still in process? I'm very interested in this project and will probably attempt to build one sometime down the line.
[Updated on: Tue, 14 July 2020 13:00] Report message to a moderator |
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The CPU board has a MMU that can access the full 16MB address space, but aside from a few extra pages for CP/M it will mostly be wasted on the 8085. My plan is to build some CPU boards that can make better use of it, though, like 80286, 68030, etc. The RAM board(s) can support both 8 and 16 bit data paths.
