Z80 NC pins

[Mark T]

Does anyone have any documentation or experience of the Z80 PLCC or LQFP packages that shows if the Pins labelled NC are not connected to the die, should not be connected externally, grounded inside the device or should be grounded on the pcb? It might make layout easier if I can use the NC pins for through connections.

Searching around I found one reference to a Z84C0040FEC in this document which would be interesting if they existed. I couldn’t find any other references.

https://www.xecor.com/product/z84c0020fec

Thanks,

Mark

[Owen Randle]

You should be able to use a DMM , on 'Diode junction' test, to check if the pins have ESD protection diodes to Ground & Vcc, - So are bonded to circuitry on the die. Or whether they measure completely open-circuit, so are unlikely to be bonded to the die.

If the same die is used in a DIL40 version as a PLCC44 package, then it is most likely that the extra 4 NC pins won't be connected to anything, and can be either left floating or connected to ground etc. if not used by other variants in the same package.

[Tom Storey]

In my experience, the datasheet should say if the pin is connected internally or if there are any specific requirements to leave it floating or tied to either rail. That's a pretty big deal really, and nothing should be left to guessing.

Certainly if I were designing something and needed to jump through a NC pin, if the datasheet and errata didn't say anything otherwise I would consider it electrically isolated.

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[Mark T]

Unfortunately none of the z80 datasheets I’ve seen even mention the NC pins. They show pinout for PLCC and LQFP but no mention of what they mean by NC. It might be as simple as not connected, but its not specified.

[Tom Storey]

The way I see it, and would design for it, is if it isnt specified, then it cant be too important.

The DIP part only has 40 pins, presumably they put the same die in the QFP and PLCC packages and have 4 physical pins left over.

If nothing says what to do with these left over pins and they are marked NC, then they cant be connected to something, otherwise they would say so. :-)

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[Петро Левченко]

> if the Pins labelled NC are not connected to the die, should not be connected externally, grounded inside the device or should be grounded on the pcb?

If a pin is marked "NC" (not connected), it means it's physically present on the housing but not connected to the chip inside. This can be verified by applying 12 volts to the "NC" pin—this won't cause any damage.

"Not connected" means the pin isn't connected. This applies to all chips, not just the Zilog Z80.

However, this doesn't mean the pin can be used in any way. It must be disconnected (+5 V, ground).

>  It might make layout easier if I can use the NC pins for through connections.

What do you mean by "use"?

четвер, 11 грудня 2025 р. о 08:04:34 UTC+2 Mark T пише:

[Alan Cox]

On Mon, 15 Dec 2025 at 19:35, Петро Левченко <lpn...@gmail.com> wrote:

> if the Pins labelled NC are not connected to the die, should not be connected externally, grounded inside the device or should be grounded on the pcb?

If a pin is marked "NC" (not connected), it means it's physically present on the housing but not connected to the chip inside. 

Frequently not the case. Many devices have NC pins that are for test purposes and if you stick 12v into them you'll blow them up.

Alan

 

[Mark T]

By use the NC pins I meant to take a trace from the next pin to it and from there to the destination or source. Or in the two other cases take a trace from somewhere else on the part via the NC pin instead of through a via to the opposite side of the board. In the end I decided not to take a chance on it and leave all four of the NC pins not connected to anything.  This is only a breakout board to test one of the parts, I can experiment with other options when I try to design an actual board with the lqfp parts.

[Петро Левченко]

I disagree with your statement. :)

There is an industry standard. If a pin is labeled NC (not connected) in a chip's datasheet, it means it's "not connected." The absence of clear instructions in the datasheet, such as "Connect to GND" or "Connect to VCC," clearly indicates that the pin is physically present in the chip's package but not connected to anything. What you call a "service pin" should be labeled "Do Not Connect" (DNC) or "Leave Floating," or even "NC" with an explanation, but it should definitely be stated in the datasheet.

I, of course, strongly recommend that the author of our thread NOT use the NC pin. I can't simulate a circuit that would answer the question: "Why use a pin labeled NC?"

P.S. Do you have a specific example? Please show me, I would be very grateful.

понеділок, 15 грудня 2025 р. о 23:01:05 UTC+2 etched...@gmail.com пише:

[Alan Cox]

On Mon, 15 Dec 2025 at 22:33, Петро Левченко <lpn...@gmail.com> wrote:

I disagree with your statement. :)

There is an industry standard. If a pin is labeled NC (not connected) in a chip's datasheet, it means it's "not connected." The absence of clear instructions in the 

 

P.S. Do you have a specific example? Please show me, I would be very grateful.

Not offhand but I've met several chips where NC pins were connected for test purposes. They did tell you the pin was "not connected" and also not to connect it though if you read carefully enough. I've been burned on this one.

Interestingly some like the 65C02 say NC is not only not connected but that you should not connect anything to it.

 

[Петро Левченко]

Got it.

Advice: No, I strongly advise against this. Time flies, and the manufacturer may release updates. If you use such a contact (NC), there may come a point where "yesterday" this contact was indeed NC, and "tomorrow" it will become DNC (Do Not Connect), and the chip will be damaged.

If you have problems with the board layout, rotate the chip 90 degrees left or right, move it to a different location, and try again.

"Discovery requires experimentation." :)

вівторок, 16 грудня 2025 р. о 00:30:30 UTC+2 Mark T пише: