The CPC printer port has only 7bit data. Which is okay for english 7bit ASCII character sets, but not enough for 8bit character sets. Also, printing bitmap graphics usually requires 8bit data (though Amstrads own printers are probably working with 7bit).
8bit Printer Port Mods
Happy Computer 8bit Printer Port upgrade (DIY)
A fine method to upgrade to full 8bit has been described in the german magazine "Happy Computer Sonderheft 4/86": Disconnect the D7 pin from ground, and wire it to the PIOs Tape WR DATA pin instead (That is, Pin 12 of the 8255 chip).
Sharing that pin will cause no problems (as long as you do not attempt to send data to the tape drive and to printer at the same time). Of course, you will also have to customize your software, or BIOS ROM, or the BIOS jump vectors in RAM, in order to take advantage of the modification.
8bit Printer Port in CPC Plus
In the CPC Plus, Bit 3 of CRTC Register 0Ch can be used as eighth printer bit. For compatibility to normal CPCs it might be possible to enable/disable this feature by a 'jumper' on the mainboard.
External Printer Ports
Reportedly, the following hardware exists:
- Communicator 104 (KDS Electronics) (1992) Reportedly: "Serial and Parallel interface" (or, actually, Mini-Max Modem)
- 8bit Printer Port (KDS Electronics) (this may be same as above)
- 8bit Printer Port (Pride Utilities) (details)
There's nothing else known about that hardware.