Pulling printer port lines low

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Captain Atari
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Pulling printer port lines low

Postby Foxie » Tue Jan 23, 2018 2:21 pm

The datasheet for the Yamaha sound chip is extremely sketchy. I assume since it says it's NMOS it should be safe to pull the lines low externally when set to outputs? It looks like it can't source any real current - but that's no guarantee that it won't get damaged when grounded. If it's really NMOS I can't see how it could get damaged. But the MFP has totem-pole outputs so who's to say the Yamaha chip doesn't? I know it's possible to set the pins as inputs. I'm wondering about what happens if you don't.

I know there are joystick to printer port adaptors, which surely wouldn't be safe if shorting the outputs wasn't allowed.

I know many other systems use open collector outputs on their printer ports. The Amiga for example, and earlier IBMs. Then, later IBM clones started using totem pole outputs for EPP/ECP stuff which made it dangerous to pull the lines low.

It also looks like the maximum sink current is 1.6mA, so I'd need to use 3.3k pull-ups at minimum. I've heard of some printers overheating the sound chip - probably because they used lower value pull-ups?

I wonder if the Firebee has the same electrical specs for its printer port?

Screen Shot 2018-01-23 at 16.47.09.png

Update: it looks like it should be safe to pull the lines low. The SMP24 driver for Cubase does a write cycle, sets the port as an input and then does a read. It finally acknowledges the read by pulsing strobe. So there is a moment in time where the SMP24 is driving PORTB with it set to outputs.
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Captain Atari
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Re: Pulling printer port lines low

Postby krupkaj » Sun Feb 11, 2018 11:37 pm

What I remember when using old PARCP, Petr recommended to set the parallel port to input. There is PAR_IN.prg in the PARCP package.

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