Elsewhere, a question:


I'm not aware of anything for the hobbyist. (For €300 you can get something.)

But this seems like a good project. Something like an Arduino Mega might do it - you need 5v interface, some 26 I/Os, and enough speed to obey the bus timing.
http://arduino.cc/en/Main/ArduinoBoardMega2560
At 16MHz clock speed, it should be possible to interface at 1MHz and maybe even up to 4MHz.

Possibly you could hook up a dual-port RAM for a faster solution.

One of OHO's FPGA modules might be useful, but I don't think they offer USB and would be significantly more expensive.

Any ideas?

I had the same thought myself. I'm surprised the market hasn't provided a reasonably-priced solution. BTW thanks for checking availability and for starting the thread. This idea deserves to be nurtured.

Dual-port RAM sounds promising, although capacity might be a problem if it's a high-density ROM you're emulating.

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Looks like a 40-pin PIC16 might have enough I/Os and CPU speed to do the trick - and therefore probably some higher-end PICs too. I would expect to find a mini dev board in DIL format, but haven't yet found one. Maybe something like this
http://proto-pic.co.uk/dl40-module-daisylink/

The Teensy 3.1 is a small ARM-based board, about $20, 5V tolerant and with enough I/Os.
https://www.pjrc.com/teensy/teensy31.html

ROM emulators were common back when software development was commonly done in EPROMs, to get a quicker cycle than removing the EPROM and putting it in the eraser, taking another one, putting it in the programmer, sending your software to the programmer, programming the EPROM, and then moving it to your board. Instead, the PC would just send the software to the ROM emulator, eliminating the plugging and unplugging, erasing, and programming parts of the cycle.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

This would be a neat project which is why I was asking! In my opinion, it would be better to use a parallel SRAM so that it can work close to top speed and only use the microcontroller for programming. I seriously doubt you could get 4MHz out of a 16MHz chip and 1MHz might even be pushing it. You could also add a backup battery to the SRAM. The problem with doing it like this would be isolating the RAM from the bus while the microcontroller was programming it (assuming the system itself is halted but still powered on). Something like a 74AC244 would work but adds about 7ns to propogation delay. I don't know how fast the SRAM would need to be to accomodate the extra delay. Physical switches like dip switches could also work to isolate everything but that might add a lot of bulk to it. In any case, it would be a neat project.

EDIT: Why the dual-port RAM? Those tend to be very expensive. One port for the system and one for programming?
http://www.mouser.com/ProductDetail/IDT/7007S20PF/?qs=sGAEpiMZZMt9mBA6nIyysK6VW3GrAnba8mt%2flbIVCXI%3d
http://www.mouser.com/ProductDetail/Cypress-Semiconductor/CY7C006A-20AXC/?qs=sGAEpiMZZMt9mBA6nIyysIT6PXTPi%2fj%2fCge8OXdnpnw%3d

Yes, two ports so you don't need to isolate. 16kx8 should be big enough - that's £5.40

Edit: oops, looks like that's only 16kbit. See corrections below.

I have such an EPROM emulator, a design by the dutch magazine Elektor in the 80ties. Attached to the parallel port of the PC.
Works fine upto 1 or 2 MHz clock speeds.
It is the usual design with TTL and 6116 SRAM.

With EEPROM and ZIF sockets I do not use it often.

(Hopefully such a parallel-port connected device doesn't demand much of the parallel port, and will work with a USB-printer adapter, for those of us who no longer have a parallel port.)

But you remind me, richarde had constructed an I2C-connected emulator called ROMBO:

which we used in the beeb816 project.

Also, I see BitWise has found a couple of pointers to contemporary industrial-strength devices:

Here's the article, from December 1989:
http://techdoc.kvindesland.no/radio/yms ... 510828.pdf

Here's some info on a successor device, from Elector of September 1992:



And another one from 2001 (serial-connected).

Cheers
Ed

Shouldn't need much to build one.

A RAM chip to hold the data
3x tristate line drivers/buffers to all allow the targets address and data lines to be isolated when changing the RAM contents
A GAL chip to arbitrate target and host access control signals
A micro-controller to load the RAM chip via USB or UART

Definely less than $300. Probably less than $30 on strip board.

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

Do you think a bare PIC could do it? It needs only to pay attention to OE (and CS) and perform a lookup.

Potentially something like ..

You could do the address mapping while waiting for CS and OE to go LO if the address is stable before these signals change.

Might work at 1Mhz.

It would be easier with a dsPIC 30F but you'd need to use a TQFP package with 64 pins or more. The port bit arrangement on the 40pin DIP 30Fs is not very convenient for mapping the data and address lines. The bigger packages, like 30F6012A, have a better port arrangement and using PSV memory makes the ROM lookup quicker.

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

In an ATMega it could look like this:
It would take a maximum of 13.5 cycles (almost 850ns@16MHz) for data to become valid after chip select was applied. You could do it only, if you run the target processor at reduced speed.

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6502 sources on GitHub: https://github.com/Klaus2m5

Thanks for the proofs of concept. A pity it takes so many cycles!

Wow, you guys are really on the ball. Too bad just a microcontroller is not fast enough.



Can you give us a link?

I think with SMD parts you could almost fit the RAM, microcontroller, and UART chip (like FT232) onto a DIP 40 breakout.