[Coco] 6820 and 6821 pin compatible?

[Joel Ewy]

I'm restoring an SWTPC 6809 computer.  I intend to run OS-9 on it.  I 
also want to try out FLEX and UNIFLEX (the latter of which I'm sure this 
computer once ran).

I've got a timer board, that I believe (judging from the OS-9 
configuration of the SWTPC emulator) provides the system clock 
interrupts.  It has a socket that, according to the documentation, once 
housed a 6820, but now sits empty.  I can probably dig up the 
documentation, but I bet somebody on this list can tell me without 
cracking a book or downloading a PDF whether or not I can just plop a 
much more readily available 6821 in there instead.

If you're interested in following my progress on this project, I've been 
blogging about it:  http://8littlebits.wordpress.com/category/swtpc/

The CoCo gets mentioned as well, and I used a CoCo emulator to make a 
boot disk image I'll eventually use when I'm ready to try and boot this 
thing up.

JCE