Abusing the IEC Port

rje · Post by **rje** » Mon Jan 18, 2021 9:57 pm

Whilst thinking about using something like pi1541 to emulate a 1581 IEC device, I started thinking about the IEC port itself, and wondering, well, I betcha it could be reprogrammed to do something it wasn't designed to do. In other words, a different protocol. That's kinda what those quick-loaders did back in the 80s -- custom programs loaded into RAM on the 1541 and the C64 for parallel or fast-serial load/save.

TomXP411 · Post by **TomXP411** » Tue Jan 19, 2021 1:09 am

You can... but why? The User port is also connected to a VIA, and it's got more pins available. Everything the IEC port can do is also possible on the User port... but without the restrictions of only one data pin.

BruceMcF · Post by **BruceMcF** » Tue Jan 19, 2021 7:24 am

Certainly. You could do an SPI interface and plug in a RPi0 supported by a Kernel vector handler on device 9. Make ATN into /SEL, DATA into MOSI and /SRQIN into MISO. As long as you refrain from using tho other IEC device numbers, you should be fine.

Lorin Millsap · Post by **Lorin Millsap** » Fri Jan 22, 2021 4:24 am

Just so you know the IEC lines have diodes and buffers on them, so while you could use them other as intended you are also limited in how the pins can be used. As a whole SPI and a few others are certainly possible. Not a lot of point in it though as there is nothing to be gained by doing so.

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BruceMcF · Post by **BruceMcF** » Fri Jan 22, 2021 1:34 pm

8 hours ago, Lorin Millsap said:

Just so you know the IEC lines have diodes and buffers on them, so while you could use them other as intended you are also limited in how the pins can be used. As a whole SPI and a few others are certainly possible. Not a lot of point in it though as there is nothing to be gained by doing so.

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The gain would be if you are using the User Port for something else and don't want to spend extra on expansion cards. If you don't have any IEC devices, it's a free port, and implementing the IEC protocol on the connecting device to make use of the free port is overkill if the connection is not going to be bussed.

Lorin Millsap · Post by **Lorin Millsap** » Fri Jan 22, 2021 1:36 pm

The gain would be if you are using the User Port for something else and don't want to spend extra on expansion cards. If you don't have any IEC devices, it's a free port, and implementing the IEC protocol on the connecting device to make use of the free port is overkill if the connection is not going to be bussed.

Just make sure that will work electrically. Basically those lines are meant to be pulled into certain states which may not be compatible with what you want to use it for.

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BruceMcF · Post by **BruceMcF** » Fri Jan 22, 2021 1:45 pm

1 minute ago, Lorin Millsap said:

Just make sure that will work electrically. Basically those lines are meant to be pulled into certain states which may not be compatible with what you want to use it for.

Well, I would assume that people would RTFM before they start breadboarding, but yes, if someone breadboards the circuit once the boards ship and the circuit doesn't work, they should RTFM at that point.