6800/6820 Address decoding help...

So I've given myself yet another project to fill my ever-lessening free time with: to write a Tek 4051 emulator. I have the ROMs (thanks again, Axel) and I've coded up a nice quick 'n dirty 6800 emulation. Now I'm working on figuring out how the CPU talks to the hardware... I have at my disposal the service manuals and a disassembly of the aforementioned ROMs. I also have a physical Tek 4051, but I'm loathe to start poking the PIAs without knowing what I'm doing, given that the screen's electron beam is directly controlled by the software, it's more than possible to burn a nice hole in the phosphor if I screw up :). The CPU talks to a set of 6820 PIAs, and from there it's fairly easy to figure out what bits hook up to what devices, given the schematics (and only slightly harder to work out how they need to be manipulated, by reverse-engineering the disassembly) but I'm having a heck of a time working out what addresses the PIAs' inputs & outputs are mapped to in the CPU's address space. The Service Manual gives a tantalizingly small amount of information about this in Vol. 1 -- it actually has a table of addresses but is very vague -- for example, I now know that addresses $878C-$878F are "Y-AXIS DA/TAPE" but I don't know which ones are for which, nor do I know whether they're input/output/both, etc... looking at where these addresses show up in the disassembly makes a few things clearer, but is not enough. So it seems obvious that the actual schematics would come to my rescue here... but I'm unable to decipher them to the point where I can determine how the PIAs are actually addressed by the CPU. I chalk this up to my lack of experience in digital logic & schematic reading. I guess what I'm asking: can someone out there help me work this out (or give me some pointers on how to attack this?). I want to learn how to do this and I need a bit of assistance... The 4051 schematics are on bitsavers (yay) at: http://www.bitsavers.org/pdf/tektronix/405x/070-2286-00_4051_Service_Vol2_M… For now, I want to concentrate on working out the display controls... the related schematic pages for these are: 4_02-03 (cpu), 4_02-10 (x d/a) and 4_02-11 (y d/a). Thanks for any advice... Josh