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)