USB To Serial Adapter

[Spikenzie] is at it again. This time he’s posted a nice USB to serial converter. He points out that you can buy a USB to serial adapter pretty cheaply, but sometimes you want this functionality built into your project enough to justify the greater cost. In those cases where you want to integrate it, this is an elegant solution. This design was made to be fairly compact and still usable on a breadboard. As usual, you can download the files from his website.  If his work looks familiar, it’s because we’ve covered his work several times before.

[via Flickr]

21 thoughts on “USB To Serial Adapter

  1. Personally, I would not even include this unless a serial port is necessary on a commercial product.
    I obtained my (working) serial dongle off ebay for about 50 cents as do many others…it’s simply not worth the effort to do anything more expensive.

  2. I wonder if you can somehow hack those PS/2 to USB dongles into one, many people have those things in drawers since they used to come with mice and keyboards, and PS/2 is serial too right?
    I always wondered what was in those things.
    But maybe they are unidirectional.

    @sephanor yeah sure, 50 cents, you know if you are going to tell a story keep it believable.

  3. For the amount of realestate available on that board, why not use an FTDI chip instead of that package? I’ve worked with LGA packages before (or something similar in this case) and they’re a royal pain in the ass to work with.

  4. $2.99 from

    I’ve only bought one of those Chinese clear-blue plastic USB-serial adaptors on eBay, drivers never worked, binned it, was somewhat disappointed because I wanted to program PICAXE chips with it, ended up spending ‘real’ money on the official PICAXE USB-serial cable and it worked on my laptop fine, except I can only get it working in one of the 3 serial ports (Windows & drivers don’t always mix very well)

  5. Radioshack sells a damn good usb converter. It’s one of the few adapters that will talk to allen-bradly equipment for me, which is a testament to how good it works. It’s the gigaware one they sell. It’s not cheap though.

  6. @whatnot:
    AFAIK, those PS/2 to USB dongles don’t really have much to them. The device itself does all the hard work. It tries USB first, and if that fails, it starts putting PS/2 signals through the USB cable. The dongle is really nothing more than a physical adapter.
    If the device doesn’t support PS/2, a USB-to-PS/2 dongle won’t work for it.

  7. This is pretty similar to FT232 based boards from sparkfun and a few other companies.

    This isn’t a converter for usb to true RS232 serial, its ttl level serial which is used to connect directly to mirco controllers and gps receiver modules and the like.

    With the converters others here a mentioning you need a MAX232 chip or similar to convert the levels back down to ttl or 3.3v even.

  8. So, he has made a serial adapter, wow!
    Most of us (well, those of us that plays with electronics), have probably done that several times. I did my first 4 years ago, and have been selling several thousands of the second.

    Anyone considering implementing a serial port on a project would obviously add the components directly to the main PCB instead of taking up space with a module like that.

    Nice work on the one-sided PCB solution though.

    Now, I’ll go to my lab and “invent” an electronic siren or blinking LED, so I can be published on HackaDay and be famous.

  9. @ mosheen,
    Do you have a part number for the unit you use from Radio Shack with Ab equipment? I used to use a cheapo blue unit from China then I bought a new laptop with a serial port but some of my techs have trouble getting the cheap dongles to work with older PLC5’s.

  10. Unused mobile phone data cables (especially Nokia, in my experience) have a decent USB -> Serial chip inside them which is pretty easy to adapt as long as you are capable of stripping a plug or two, tracing pinouts and soldering / crimping an adapter to the phone end.

    By no means as elegant as this, but good in a pinch.

  11. We should all go to JB’s blog and talk about his amazing projects… oh… he doesn’t have a blog to talk about the projects that he doesn’t do.

    @JB, yes, you are awesome. In fact you are so awesome that I am surprised you haven’t picked up on how you are way too cool for this board. Do you have a blog with any/all your projects on it? I would love to see even one of them just so I can bask in your awesomeness.

  12. Man, if someone can recommend some decent low-cost machined pins or lead frame pins for something like this where you don’t have a 1,000 pin minimum order; let us know. Yet another author that makes a 0.1″ spaced breakout-type board and doesn’t address where to get the dang pins. SparkFun has a zillion of these little breakout boards and even they don’t address where to get pins. I end up using some copper wire – not a really good solution.

  13. @JB on his page he explains that he intends to use it for breadboarding.

    To use it as a module is a little much I agree. It would be feasible if the module is very common and lower cost than implementing yourself (zigbee, etc.). But now its easy for him to copy and paste this working circuit/layout into a new design.

  14. female usb to male ps/2 adapters are sold at 1000 for 20 bucks online, or a quarter each, but i would probably buy 1000 for $20 and use them to make something awesome. now usb to serial would come in handy for this ancient laptop i have that has IR port, Serial port, pcmcia, etc, but no usb ports and i seem to have lost the cd rom drive. no usb ports because it is ANCIENT and it runs windows 95 but i have no way of getting data onto it, and it really needs some drivers for the ethernet card, or its useful only for minesweeper and solitaire. if i ever find a way to interface with the hard drive or something, it’s getting something small and fast like maybe puppy linux, although this is one of those times that even puppy linux cant run from the RAM because its only got like 32mb.

  15. I guess you are right about the ps/2 thing not going to be of much use, and I wasn’t thinking anyway since what you would need is something in reverse, a device with a PS/2 connector and a dongle to make it USB, I’m not sure that even ever existed.
    But talking of PS/2, I understand that that is serial so maybe when you have a computer with an unused PS/2 port and no COM port you could somehow use the PS/2 port as a replacement for the missing COM one, but I guess you’s still only have input then, and it would require you to hack together your own driver, and get it to be accepted by the OS you are using too, so I’m not sure it would be worth all the work, except when you really really like a challenge.
    Thanks for setting me straight rd

  16. @Vonskippy:

    If you’re serious, a basic arduino has all the necessary hardware (both USB Device port and a serial port). One just needs to program it a bit, as some kind of data pump.


    I once managed to stuff RH7 (stripped down to 40 MB, graphics weren’t the goal) to a similar laptop: Pentium1, 100 MHz, 500 MB hdd, some small amount of memory (perhaps also 16 MB) and a working floppy. Difficulty: RH7 installer needs a lot more RAM just to uncompress own packages and has no alternative of installing from floppy. Therefore, must code a custom installer. That was a decent adventure, and the thing was heavily used afterwards almost for a year; however now I would begin from busybox and its built-in tar — perhaps less coding involved.

    Your dinosaur could be saved by a null-modem cable. Win95 probably doesn’t contain intersvr.exe and interlnk.exe, which surely were available in DOS 6.22. However there is the standard “Direct cable connection” driver.

    Interfacing those disks is another story. I once fried a 20 MB disk from Toshiba 1000LE (a 10-MHz 8086! MS-DOS 3.2!! 16 shades of gray!!!) just by trying to access it from a recent motherboard. The wiring adapter was used earlier with a recent 2.5″ disk so it’s not an electrical problem. Just to be sure, I found the official pinout of that bastard beforehand – no significant differences from contemporary IDE. Ergo, mere ATA protocol differences killed it :( Now the thing boots from floppies only. Shall I upgrade the BIOS chip to a Flash one and start reverse-engineering its contents so that newer disks are supported? :) A ludicrously messy job, and of little value: even arduinos now have more processing power.

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