Further to the query (issue 69) on connecting a LX180 printer to a BBC or Newbrain AD, there are three points to consider: first, the LX180 is not centronics standard. The data lines are inverted, i.e. logic is 0 volts.
Secondly, all lines should be buffered to prevent possible damage via paralleled pull-up resistors.
Lastly, the LX180 is a synchronous printer - characters must be ready to be sent at 5.5ms intervals or less - as requested by the printer.
The first two hurdles can be overcome with a small circuit inserted in the cable and powered from printer or computer. But the third could be a problem if the computer does not respond to AC knowledge from the printer (called PEC by the LX180 - Strobe is CI).
I have also just interfaced a LX180 to my Dragon. The Dragon is one computer which does not respond to ACK. My LX180 has a 256 character buffer and the interface consists of the data inverters line buffers and two one-shots. The first of these produces a busy signal and this is gated with the buffer full signal to produce BUSY which goes to the computer. The second one-shot stretches the very short STB of the Dragon to ensure the LX180 does not miss a character.
An easier way, which will work without the buffer board as well, involves setting a flip-flop with STB from the computer and resetting it with ACK from the printer. The flip-flop output is used to control the computer BUSY line.
Of course, if your computer does respond to ACK, there is no problem.
18th August 1984
