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Author Topic: ft245 (232 or 2232 in 245 mode) interface to 68000 bus?  (Read 178 times)


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ft245 (232 or 2232 in 245 mode) interface to 68000 bus?
« on: November 03, 2019, 12:45:38 PM »

Anyone have advice, or an example schematic connecting the 8 bit data bus end of an ft245 as an I/O device to a M68000? (or 6502 for that matter.)

I'm particularly interested in what buffering and what handshake signals, signalling DTACK and IRQ, would be needed, and using whatever modes would enable bi-directional buffering transfers rather than just bitbang and at the highest data rate possible.

This would be to allow me to connect a modern USB computer to an older 68k machine and be able transfer data back and forth.

I realize that on the 68k side I'd need some tri-state logic to detect the I/O address and enable the 68k data bus lines, as well as 5v<->3.3 conversion, but I'm less sure about the handshaking logic to the ft245 side. Ideally I'd like to be to write code on the 68k side to MOVEP bytes in a loop, one at a time, in or out and not potentially lose them. That is to use the internal buffer that the ft245 chips do, so on the modern PC side I could use the driver and read the same byte stream sent by the 68k or write a stream of bytes and have it delivered to the 68k side.



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Re: ft245 (232 or 2232 in 245 mode) interface to 68000 bus?
« Reply #1 on: December 03, 2019, 04:13:26 PM »

Hey there, mc68000 - Fellow retro computer engineering hobbyist here. :)  I don't have any schematics to share, but I have a few comments:  First off, the FT245 is a 5V-capable device.  Supply 5V to VCC and VCCIO.  No level translation needed.  Regardless of interrupts, you'll still want a way to poll the FT245 for status, as sending a block of data with interrupts between every byte would be horrendous, performance-wise.  So, map the TXE# and RXF# using a tri-state buffer at some other memory location.  You may have other peripherals with similar needs, so you can probably find things to do with the remaining bits.  /DTACK is a larger topic, but if you're using all reasonably fast peripherals and memories you might get away with simply tying /DTACK to ground, as many others have done in simplistic designs.

If your memory map and selection of peripherals are complex, however, you might bite the bullet and throw a cheap PLD into the mix.  A little will go a long way.  I recently did a 6502 build and fit all the address decoding into a $1.50 22V10 chip.  They're still available from Digikey, and you earn extra 'retro' bonus points by programming them in the ancient CUPL language. ;)