Msg: 5416 *Conference*

09-12-94 23:46:50

From: COMET _

To : ANTHONY FEST

Subj: REPLY TO MSG #5396 (PRINTERS)

Parallel printers are just as popular as ever; many printers have BOTH a serial
and parallel port.  A print-buffer (external) can work with ANY computer that
follows the Centronics parallel standard or the RS-232C serial standard
(otherwise, some cabling is required).  You are correct in surmising that you
cannot use your modem AND serial port on the Model T simultaneously; both the
internal modem and the serial port use the same UART (Universal Asynchronous
Receiver/Transmitter chip.  The purpose of the UART is to convert the serial
start, data, parity, and stop bits to parallel data verifying proper code
transmission, parity, and stop bits (for the receiver function.)  The UART also
converts data from parallel to series (for the transmitter function.)  This is
necessary since the CPU (and other chips) operate on 8 bits at a time, and the
modem (and serial port) send/receive data a single bit at a time.  You can see
why the parallel transmission scheme hasn't gone out of style (although it has
to have a thicker cable).  Using an external parallel-to-series printer buffer,
as I do, allows one to use the parallel port of the Model T to drive a seriial
device, such as a CRT.  Indeed, I do get 80 columns output whenever I "print"
from the Model T.  (Or 132 columns, if I set my VT320 in that mode.)  I am
using a Practical Peripherals Mini P/S 512 for this purpose.