Ritengo molto interessante evidenziare quanto avvenuto a metà degli anni ’80 facendo riferimento a quanto citato da un documento disponibile in Internet di cui sono venuto in possesso. Si tratta di una insieme di suggerimenti e modifiche raccolte in un articolo chiamato “A Xerox 820-1 Compendium” e pubblicato dall’agosto 1985 sulla newletter dell’AMRAD (Amateur Radio Research and Development Corporation) e successivamete sulla rivista QEX dal Giugno 1986 al Gennaio 1987. La immagine seguente è il  frontespizio della rivista QEX con la mainboard 820-1:

All’inizio del primo articolo è riportato quanto segue e dimostra il motivo dell’interesse per questa scheda da parte di radioamatori per la realizzazione di sistemi di trasmissione dati via etere.

History

During the summer of 1980, Digital Computer of Texas marketed a Z80® computer in kit form and named it the Big Board, or BB-1. The hardware was designed by Jim Ferguson of Ferguson Engineering, but by late 1980, Xerox had bought manufacturing rights to the circuitry. The company rearranged the circuits on a multiple-layer board and renamed it the 820-1. (Beware of hidden connections; some of the +5V and ground lines are now sandwiched inside the fiberglass board!)  The circuitry remained identical, but all IC numbers were changed.

By 1982, Xerox marketed the 820-2. It sported a faster clock speed, double density and offered a possibility of adding a 16-bit co-processor. Although there are similarities between the 820-1 and the 820-2, there are also a number of differences. This Compendium will deal exclusively with the 820-1.

As obsolete 820-1 boards were removed by Xerox and replaced with the newer 820-2 board, the 820-1 surfaced on the surplus market.    By October 1982, these surplus boards were selling for $435 each, and as of July 1985, they could be found (untested) for as little as $50.

Ho raccolto le 6 parti in un unico documento, ed al momento il sito dal quale ero risalito alle 6 parti non è più disponibile.  Alcuni numeri della rivista QEX sono ancora disponibli a questo indirizzo. Di seguito riporto l’elenco dei numeri in cui sono apparsi i 6 articoli:

A Xerox 820-1 Compendium Part 1:   1986  7, Jun
A Xerox 820-1 Compendium Part 2:  1986  7, Jul
A Xerox 820-1 Compendium Part 3:  1986 14, Aug
A Xerox 820-1 Compendium Part 4:  1986 10, Sep
A Xerox 820-1 Compendium Part 5:  1986 12, Dec
A Xerox 820-1 Compendium Part 6:  1987 7, Jan

Il documento mi è stato estremamente utile quando ho avuto la necessità di creare una uscita video composita dai segnali Video, Hsync e Vsync separati forniti dalla scheda. Riporto di seguito il relativo dettaglio:

… produce composite video as follows:

1. Cut the trace from U36-2 to U53-5. This changes the vertical frequency from 58.76 Hz to 59.96 Hz. This is a very simple modification, and can even be done on the top of your board if you already have it installed in a case.

2. Remove R60, 150 ohms, between U106 and U117 near U105.

3. Cut trace from U117-10 near the pin on the bottom of the board.

4. Solder a 39-ohm resistor (use insulation tubing) between U117-10 and the right hole from where R60 was removed (it goes to J7-5).

5. Using one lead of a 100-ohm resistor, jumper J7-3,4,5 together and connect the other end of the resistor to J7-10 (ground).

6. Reverse the horizontal sync polarity by cutting trace to U15-4 (on top of board) between that pin and the adjacent feed-through. Route a feedthrough to U15-13.

7. Check your sync pulse lengths. HSYNC should be 5.0 Ms and VSYNC should be 400 us.

When the Xerox board is converted to produce composite video, the display may “swim” slowly from side to side. This is because the vertical oscillator frequency is 58.76 Hz rather than 60 Hz, and this oscillator “beats” with the 60-Hz line frequency, causing the swimming video. This can be fixed with the following modification:

You will now have composite video on J7-3,4,5 and ground on J7-8,9,10. Dunbar advises that if you have dc coupled input on your monitor, you may wish to couple the video through a 100-/xF capacitor shift in the black level. 

 

 

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