In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

John,

dealing with an "old", or "classical" Centronics parallel port is not really difficult.  It has the following pins:

1 - O - STROBE*
2 - O - D0
3 - O - D1
4 - O - D2
5 - O - D3
6 - O - D4
7 - O - D5
8 - O - D6
9 - O - D7
10 - I - ACK*
11 - I - BUSY
12 - I - PE
13 - SEL
14 - O - AUTOFD*
15 - I - ERROR*
16 -O - INIT*
17 - O - SELIN*
18...25 GND

Where "O" means output, and "I" means input (when you "look" into the PC's parallel port). An asterisk (*) marks negative logic.

So, here is a rough scheme for a simple parallel to serial conversion in kinda BASIC dialect:

WaitForNextChar:
Let BUSY = low

WaitForStrobeLow:
If STROBE* = high then goto WaitForStrobeLow
Read D0...D7 from the port into a byte variable
Let BUSY = high

Pass byte variable to a serial transmitter VP to send it to the serial printer

WaitForTxDone:
If SerialTransmit <> Done then goto WaitForTxDone

WaitForStrobeHigh:
If STROBE* = low then goto WaitForStrobeHigh

Goto WaitForNextChar:

It depends on the serial printer, if there is a handshake required on the serial side. So you might check what kind of handshake the serial printer can handle. Many printers make use of the RTS/CTS lines which is pretty easy to implement. Others use an XON/XOFF protocol which requires some more tricks.

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