This 8bit Printer Port expansion is described in the book Maschinenspracheprogramme und Hardware-Erweiterungen für Schneider CPC's by Paul Bauriedl. It only connects to the centronics port of the CPC and doesn't require internal modifications.
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