Serial (RS-232) communication channels usually depend on certain control codes, such as XON/XOFF, being sent as part of the data stream. A PostScript job on an RS-232 channel may include additional control characters to perform these tasks:
When a serial printer receives a data stream containing control codes, the printer's communications interface traps and removes the control codes. When this is the case, it means that the serial channel is not binary-capable.
If you direct binary data to this type of communication interface, some data values will be misinterpreted as control characters and erroneously stripped out. This, obviously, will affect the printer output, and cause distorted or garbled printing.
Also, a serial channel may be configured to use less than 8 bits of data per character.