This 8bit Printer Port expansion is described in the book Maschinenspracheprogramme und Hardware-Erweiterungen für Schneider CPC's by Paul Bauriedl. It connects externally to the 7bit printer port (and the 5V power supply) of the CPC and doesn't require any internal modifications to the CPC.
Description
In this design, only the lower 4 bits of the printer port are directly connected to the printer. The upper 4 bits are generated with the help of a latch (LS 75). If you want to print, you have to send the upper 4 bits in D0-D3 and set D4 to high to buffer the values in the latch which is connected to D4-D7 of the printer. The rest of the data can be sent by the normal printer routine of the firmware (with D4 set to 0 to disable the latch).
Schematics
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Listing
10 ;Patch fuer 8-bit Centronics 20 ;direkt unter HIMEM (+Floppy) 30 ; 40 indjmp: equ #bdf1 50 ; 60 org 42619-40 70 ;Initialisieren 80 ld hl, (indjmp+1) 90 ld (retour+1),hl 100 ld hl,bit0 110 ld (indjmp+1),hl 120 ret 130 ; 140 ;neue Ausgaberoutine 150 ;ist ueber Druckerindirektion 160 ;#BDF1 eingeschleift 170 ; 180 bit0: ld b,#ef 190 push af 200 srl a 210 srl a 220 srl a 230 srl a 240 out (c),a 250 set 4,a 260 out (c),a 270 res 4,a 280 out (c),a 290 pop af 300 and #0f 310 ;Jump wird beruecksichtigt 320 ;je nach System! 330 retour: jp #07f8