CREATE A TWO-HEADED PRINTER CABLE

Double your printing ease, double your fun

by Hans Hsu

Are you a two-printer owner? Connect both printers to your Atari and switch between them effortlessly with this homemade cable. As written, the design requires an Atari 850 interface. But the more adventurous should be able to adapt this principle for other set-ups. You will also need some soldering skill and a few bucks worth of parts.

The following article assumes knowledge of hardware. We have taken every step to ensure that the information contained herein is correct, but Antic cannot take responsibility for, nor give assistance on, readers' hardware projects – ANTIC ED.

I recently added a second printer to my Atari system, giving me both daisy-wheel and dot-matrix capability. But since the Atari 850 interface provides only one parallel port, I found myself in a tangle of interface cables, constantly uplugging one printer and plugging in the other. I quickly grew tired of all this and decided to invent a better way.

I found that with a double-pole, double-throw (DPDT) switch and a little soldering, I could convert my two printer cables into one double-headed cable. Each of the heads plugs into a printer and the tail plugs into the 850's parallel printer port. A switch at the neck selects the printer for output. Now, when I want to change printers, all I do is flip a switch.

SOME THEORY

While all parallel printer ports have ten or more lines, only two are control lines which are significant to our project: STROBE and BUSY.

The STROBE line is used by the computer to tell the printer there is valid data on the line worth grabbing. Without the STROBE line, you could send data to the printer forever and nothing would get printed. The BUSY line is used by the printer to tell the computer it's ready to receive data.

NOTE: Pinout locations for Centronics plugs vary between different printers. Check your printer manual carefully.

Now, we use the DPDT switch to send these two control signals only to the printer we've selected. So although both printers receive the data, only one gets the control signals and only one prints the data.

MAKE A CABLE

In all, you will need one D15 plug to connect with the 850, two Centronics standard plugs for the printers, one DPDT switch, and enough ribbon cable to reach everything. You might also need a 1.5-inch by .5-inch piece of perforated particle board and a plastic 35mm film canister. Since I already had two printer cables, I did not have to start from scratch. I cut my two cables in half and kept the two Centronics plug ends and one of the Atari plug ends.

Solder the data lines of the three cables together. Solder the STROBE and BUSY lines to the DPDT switch. See Figure 1. Mount the DPDT switch on the .5-inch side of the perforated vector board, and make sure the toggle of the switch protrudes past the .5-inch edge.

Now cut a vertical slit in the film cannister, just wide enough for all three ribbon cables, and insert the vector board with the ribbons trailing out from the slit. Cut a small hole in the canister lid for the protruding toggle-switch and slip the lid back onto the canister. See Figure 2.

Assuming you have made all the right connections, you should now be the proud owner of a two-headed printer cable. To test it, connect the cable to both printers and your 850. Make sure both printers are online then print something. One of the printers should respond. Now flip the switch to send something to the other printer. Note the position of the switch and mark it accordingly.

Hans Hsu sent in this article from England where he recently received an M.A. in Computer Science from Cambridge University. He designed JaneCalc for Arktronics Software and was a development engineer for Hewlett-Packard.