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Author Topic: C64 diode bodge  (Read 1539 times)

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Offline ascommTopic starter

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C64 diode bodge
« on: November 09, 2019, 10:24:32 pm »
Hi

Are there any Commodore 64 gurus here? I was wondering what's the purpose of this C64 motherboard's four 1N4148 diode bodge? Anyone know anything about it?

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Offline Quarlo Klobrigney

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Re: C64 diode bodge
« Reply #1 on: November 09, 2019, 10:48:47 pm »
Looks like diode clamping of the inverter's open collector output. One diode to ground another to VCC. A no schematic opinion. Which U number is it? Is it the U8 or is it a different reference number?
« Last Edit: November 09, 2019, 10:50:38 pm by Quarlo Klobrigney »
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Offline ascommTopic starter

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Re: C64 diode bodge
« Reply #2 on: November 10, 2019, 07:29:41 am »
The diodes are connected like this to data and clock lines.

The schematics is snipped from https://myoldcomputer.nl/
« Last Edit: November 10, 2019, 07:35:43 am by ascomm »
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Offline thinkfat

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Re: C64 diode bodge
« Reply #3 on: November 10, 2019, 07:53:00 am »
The diodes clamp the signals to the supply and ground rail. Looks like somebody tried to curb some voltage overshoot due to ringing. Strange for an open collector driver, though.

EDIT: no, not ringing. I see those signals going to external connectors, user port and serial port. Likely, it's just a protection for the CIA port.
« Last Edit: November 10, 2019, 07:58:15 am by thinkfat »
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Offline Chris_Walch

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Re: C64 diode bodge
« Reply #4 on: November 10, 2019, 12:39:03 pm »
The diodes are there to protect the input lines of the serial bus, they are connecterd to the OC drivers 7406 of the output lines.

Quite a common "upgrade" on older C64 boards, later revisions do come with these protection diodes on board.
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