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I want to do a custom PCB based on an already made one here.

There are many exclamation marks used in the schematic. I think they mean "inverted signal (not.)" Is that true? If so, what is the best practice to implement that on a PCB? A simple transistor?

enter image description here

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  • \$\begingroup\$ For later reference, in this case, I think the exclamation marks denote a different signals with similar functionality, so it is like denoting !RTS is input and RTS output, or so. \$\endgroup\$ Commented Jun 18, 2023 at 11:51

2 Answers 2

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This, typically, means the signal is active low. That is, when the signal is 0, it is asserted. This is often written as a bar over the signal name, but the schematic capture tool might not be able to render this.

A common example is a reset pin - when this is 0, the component is held in reset. You might see this on the schematic as:

  • !RESET
  • \$\overline{\textrm{RESET}}\$
  • RESETn/nRESET
  • #RESET

In the RESETn/nRESET example, the n signifies the active low semantic.

The driving source must have the correct semantic. If it does not, you need to invert it yourself - either a simple MOSFET/transistor or, for larger counts like this, a NOT gate (e.g. a 4049) may well be more space and BOM efficient.

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    \$\begingroup\$ Sometimes also shows as /RESET or \RESET, and there are probably quite a few other conventions. \$\endgroup\$
    – jcaron
    Commented Jun 5, 2023 at 16:41
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    \$\begingroup\$ It is often denoted with a hash too: #RESET, which I find really quite ugly. \$\endgroup\$
    – pfabri
    Commented Jun 5, 2023 at 16:53
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    \$\begingroup\$ Be sure you understand the signal flow of this circuit before you assume that a signal is being inverted. After a quick glance, I'm not so sure that an inversion is happening by say, U5, for example. \$\endgroup\$ Commented Jun 5, 2023 at 17:03
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    \$\begingroup\$ Strongly related! \$\endgroup\$
    – pipe
    Commented Jun 5, 2023 at 17:08
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    \$\begingroup\$ @MohamedAbduljawad Note that on your example, !RI is in a typeface suggesting it's been placed by a CAD package, while RI is part of the graphic representing the chip footprint. If you refer to the chip datasheet, you will probably find that it is active low, and you also need to consider the behaviour of any level shifters because of the RS232 signalling conventions. In short, the whole thing is a bit of a mess and a lot depends on how well you trust that the designer has presented you with a tested circuit. \$\endgroup\$ Commented Jun 6, 2023 at 6:04
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It does mean that the signal is active low.

It does not mean you need to invert it.

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