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I have removed a chip to reseat it, but sadly I forgot how it was placed on my C64 (breadbin).

I assume its text should be in the same direction of all the other chips on the board, but now I don't feel safe turning it on again and I wanted to ask.

Here's a picture.enter image description here

In the worst case scenario, what can happen if I put it upside down and turn on the machine?

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    Does the socket have a Pin 1 identifier (the end notch or corner dot)? Usually that's present to help guide the fitment of the IC. In the worst case scenario, powering on with incorrect alignment would destroy every IC on the board and initiate a fire. Unless there's something worse that I've overlooked. Jul 27 at 15:02
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    @TobySpeight Releasing toxic magic smoke, which OP inhales?
    – wizzwizz4
    Jul 27 at 15:25
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    Please clarify in post which IC you removed and is in doubt about. (In my answer below, I am assuming the original ROM into the daughterboard.)
    – Retrograde
    Jul 27 at 16:45
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In general, IC orientation is designated by the notch in the centre on one end of the IC. (Don't make assumptions based on the text direction, although typically it will flow left to right from the notched side.)

And yes, typically, board designers will orient ICs in the same direction(s); but again, don't make assumptions.

On the PCB, the notch is typically marked on the silk screen as well.

If mounted correctly, sockets will also have a notch showing the intended orientation.

I am assuming you are asking about the orientation of the original ROM inserted into the socket of the daughterboard, mounted in place of the original ROM.

Since this looks like somewhat of a homebrew board, it's probably extra wise not to make assumptions, but check that the upper, right pin (VCC/+5V on both the original ROM and EPROMs) of the ROM is connected directly to the upper, right pin of the secondary EPROM as well. You can use a multimeter in continuity mode, or trace it visually on the daughter board.

Inserting it the wrong way around is likely to damage the part itself; probably the rest will survive, but worst case other parts connected to the address/data buses may be damaged.

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    As a board designer, I generally pay no attention to the direction the IC is oriented; only to what makes the layout easier. I suppose that wasn't so big of a concern in the 80s when you were operating at such low frequencies that you can just meander your signal traces around wherever without worrying about signal integrity, though.
    – Hearth
    Jul 28 at 3:09
  • @Hearth but that generally means ROM chips in a row will be oriented the same way around so the address and data lines don't have to cross over from one chip to the next. Here we see a board with the original ROM chip on it, which almost certainly will be oriented the same way as it was on the main board. Jul 28 at 6:47
  • Sorry if I come back to this but... If I measure continuity between pin1 (and 2, 3, etc) of the real chip and pin1(and 2, 3, etc) on its own socket... I can safely assume that I should orient this little daughterboard in the same direction of the only original chip, right?
    – Pitto
    Aug 3 at 11:30
  • The pinout of the original ROM and the EPROM is likely not completely identical (as I recall, the EPROMs typically have a few more pins). However, these models (ROM+EPROMs alike) generally have +5V on the upper, right pin, so for a sanity check I'd just confirm that there is continuity on that pin between board socket, ROM on daughterboard and EPROM on daughterboard.
    – Retrograde
    Aug 4 at 22:16

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