FRS' MSX page WIP

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By sd_snatcher

Prophet (3366)

sd_snatcher's picture

15-03-2020, 15:17

It's not suffice if only one side of the communication is open-collector, gdx. All outputs connected to an open-collector bus must be open-collector. It then works like a wired-AND circuit.

My best advice is that you seek some online tutorials about open-collector buses. And wired-AND circuits too. They'll explain everything in much greater detail than I would be able to do in a forum.

By gdx

Prophet (3811)

gdx's picture

16-03-2020, 13:25

The right Megadrive controller schematic is:

{image-share.com has been hacked, so I deleted this image -hamlet}

Do not confuse open-collector with Pull-up. I invite you to search the characteristics of open-collector components.
http://datasheetcatalog.com/datasheets_pdf/7/4/L/S/74LS09.shtml
http://datasheetcatalog.com/datasheets_pdf/7/4/L/S/74LS07.shtml
(Normally in electronics we can connect an input to an output, two inputs but not two inputs. This is the reason that makes you put diodes is not it.)

By gdx

Prophet (3811)

gdx's picture

16-03-2020, 12:08

image-share.com seems to have been hacked. I put again the image.

Quote:

Normally in electronics we can connect an input to an output, two inputs but not two inputs.

...not two outputs.

By Grauw

Ascended (9379)

Grauw's picture

23-10-2020, 20:20

sd_snatcher wrote:
gdx wrote:

Why do you think diodes are needed here? Controllers with chips for autofire don't use diode.

http://www.msx.org/wiki/Panasonic_FS-JS222

As I explained, the diodes are needed because of the risk of bus contention.

Without the FS-JS222 schematics, it’s hard to know how it’s implemented. But judging by the PCB picture, I suspect that it uses the transistors for this protection job.

And they are not connected as long as the buttons aren’t pressed, so there can only be a short when they are pressed and the auto-fire is active. MegaDrive controllers output 5V when the buttons aren’t pressed so the situation is different.

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