"Unconnected pin" in PSpice

[DigiTan]

I'm trying to simulate a design that leaves one pin of a 555 timer unconnected. Other CAD programs like Eagle make this easy for you by offering a "No connect" part in the library. PSpice's? It's doesn't do crap.

So every time I try to simulate, it gives me the error: "Unconnected pin, no FLOAT property or FLOAT = e X1 pin 'TRIGGER" and leaves an annoying green marker on top of my No Connect.

So I figured, no big deal. I Googled **broken link removed** and followed the instructions exactly. It didn't fix any thing.


Two questions: why doesn't the "NC" part do what it says? And how can I simulate with a pin unconnected? And forget grounding the pin through a 1-billion ohm resistor; the method works but it seems totally inappropriate and looks tacky on paper. How do I fix this?

 

[audioguru]

Which pin of a 555 do you want to leave disconnected? Pin 5 is the only one if the supply has a good bypass capacitor or regulator.
Sometimes pin 7 is not connected to anything.

 

[DigiTan]

I'm trying two variations of a ramp generator. The 1st one is monostable and this one claims to be astable and leave the trigger and reset pins floating. I'm not certain if it's a diagram error or not. This isn't the first time I've needed unconnected pins in PSpice, so I figure I'll get it out of the way.

 

[audioguru]

A 555 doesn't work with the trigger pin open. It is the very sensitive base of a a PNP darlington that is part of a differential input and therefore it will float high if it doesn't pick up interference. The 555 does nothing when its trigger input is high.

A 555 might be falsely reset if the reset input is open since it is the base of a PNP transistor. It should connect to the positive supply if it is not being used.

 

[DigiTan]

But still, how do I solve the "unconnected pin" problem?

 

[audioguru]

If you disconnect a pin that is supposed to be connected then I don't think PSpice will tell you what error will occur.
Which pin of the 555 are you leaving disconnected?

 

[DigiTan]

Pin 4. But, the 555 isn't even the point here, I'm just trying to get PSpice to allow disconnected pins. A later part of this design will use 7400 series chips where more than half the pins will be unused. Somewhere along the lines I need do know how to do this.