The printer driver is a very simple device which just sends the characters to a printer (or other device) using the built in centronics type parallel interface.
Only one channel at a time may be open to the printer driver, if an
attempt is made to open a second channel then an error
(.2NDCH) will be returned. A channel can be opened by giving
the device name "PRINTER:", any filename or unit number is
ignored.
Having opened a channel characters can be written using either the single character write or the block write function call. The characters will be sent without any interpretation at all, and all 8 bits are sent.