Trying to rebuild the FDC card by my xerox 820-II motherboard, I have
discovered the presence of an IC named 733w01522 (U1) on the photo of the
original PCB which code I cannot read on the schematic not very readable
reported at the end of xerox820-II service manual.
Sorry, I mistakenly indicated a 14-pin TTL chips even though it was 16 pin.
Thanks for contribution of Chuck Guzis from the VCF he told me that the
733w01522 is a PROM and it is equivalent at 74S288 but also to others IC as
reported here
http://www.hobbyroms.com/proms.html.
U1 (74S288) and U2 (74S174) together constitute a "finite state machine" (so
they told to me cause I'm not so expert) whose purpose is to provide a read
clock and a read data to the chip WD1797 from raw data received from the
drive.
Then I need to discover the code that the chip could have (without having
the original chip programmed with me!).
Upon the recommendation of Chuck Guzis I found the code listing on page 12
of the manual of the FDC1797's application notes. At this point, trusting to
this code I'd need to buy a programmed 74S288 (or equivalent).
After all this I was finally trying to figure out if it sould be better to
use of a schematic published on an old issue of Microcornucopia:
https://www.dropbox.com/s/4f8kblogrlzdr9m/FDC_from_Micro_Cornucopia_#22_Feb8
5.pdf.
Who wrote that article decided to build itself a FDC controller card for his
xerox 820-II extracting a block of its schematic from a Kaypro-II's
schematic really making very few changes and using a FDC WD1793 + WD9216 for
external separator data.
Due to its small number of components, and having already a WD1797 I'd just
need of a WD9216 (i hope to find it on the Internet yet) and wiring it would
be maybe more simple.
Last suggestion (always from the so expert Chuck that I thanks so much)
would be to use a WD2797 instead of WD1797, but I'm not a designer and while
i can understand something of all those things it's very hard for me to
change schematics or adjust components.
I hope to be able to buy a ready Xerox 820-II FDC board even I know that
it's not very easy to find. Thanks anyway for all the support that was given
to me.
Regards
Enrico - Pisa (ITALY)