# Parallel port as TTL serial

Is it possible to use a parallel port (a real one, not an USB converter) as a TTL serial port equivalent, for a maximum speed of 115200 baud ?

I'd like to talk to a router's serial port but I don't have any TTL level converter, and buying one is not an option (I can't afford to "waste" 20 bucks on a stupid "hey let's reverse engineer this router!" idea).

• @WoutervanOoijen I was talking about buying one already made, which is about 20 bucks with shipping. Of course it's cheaper if you buy the parts and assemble it yourself but I prefer to avoid that. – user40669 Nov 14 '14 at 20:30
• @WoutervanOoijen but since you say it's impossible, you may as well post that as an answer. Thanks. – user40669 Nov 14 '14 at 20:31
• It probably is possible to bit-bang serial at fairly high baud rates on a true localbus parallel port, if you write your own bare-metal program dedicated exclusively to that goal. It is probably not possible under a general purpose desktop O/S. – Chris Stratton Nov 14 '14 at 20:43
• While a converter could well prove to be as simple as a pair of transistorized (or IC) inverters. – Chris Stratton Nov 14 '14 at 20:43
• As @ChrisStratton says, a couple NPN transistors, four resistors and a diode would 99.9% likely work (needs a 5V supply from somewhere). – Spehro Pefhany Nov 14 '14 at 20:57

So although it would be possible, it's really not worth the huge amount of effort, when all you really need is a little USB dongle that you can pick up for $3 on eBay. Using the parallel port as a serial port: not without adding parallel-to-serial conversion hardware. A serial-port-to/from-TTL converter can be as simple as a max232 chip and a few 1uF capacitors. If that costs you 20 bucks you are visiting the wrong shops. Did you ever check www.dx.com? Free shipping... http://www.dx.com/p/usb-to-ttl-cp2102-serial-module-red-309988 http://www.dx.com/p/3v-5v-serial-port-rs232-to-ttl-converter-module-340324 Note: As an USB-to-serial converter the CP2102 (and everything else that isn't an FT chip) doesn't have the best reputation, but is is cheap... • I ordered some small prototype boards from dx -- maybe$20 out of a much larger order. For some reason, just those got hung up in customs, and took about 4 months to reach me. dx was communicative, but it was a bummer, and I had to order much more expensive boards in a rush to equip a summer course I was teaching. – Scott Seidman Nov 14 '14 at 20:40