Why are UDN chips so rare when...

[RayB]

all these guys make them:

http://www.chinaicmart.com/buyer/search.html?keyword=UDN7180a&searchtype=1&sel1=1

[Pinball Wizard]

What are these chips used in?

If you want to talk about rare chips try and find the original chip for my Say-It-Again board in my Centaur. I gave up and are eventually just going to build a replacement board for it.

[RayB]

The UDNs control segments in high voltage glass displays like you find in pre DMD as pins.  I learned from someone that though there are companies claiming to make them, they are mostlymcompletely unreliable or don't work at all.

[Q*Bert_OP]

The UDNs control segments in high voltage glass displays like you find in pre DMD as pins.  I learned from someone that though there are companies claiming to make them, they are mostlymcompletely unreliable or don't work at all.

Correct. These chips are often counterfits, and usually don't work. Same thing with the 6821 PIAs...good luck finding these new.