Parallel Port Break Out Board (BOB)




About: I was pfred1 but moved, changed my email address, and lost my password. I suppose worse things could happen.

Intro: Parallel Port Break Out Board (BOB)

Ever want to interface with a computer? I don't mean in the regular way either, I mean the real down and dirty strip wires and plug stuff in kind of interfacing! Yeah! No? Then this Instructable isn't for you and you should move on now.

Oh you're still here? Good! In this article I'd like to present a project where I did just that. Now I should say right up front that you can just buy these things and they're called Break Out Boards (BOB) and one will set you back anywhere from $25 to say maybe $50 or more depending on the features, or how greedy the seller is etc. My board cost me about $6 to make but I had some of the most expensive components on hand to work with. Namely the barrier terminal strips, which if you're creative you don't even really have to use.

These boards aren't much more than glorified terminal strips anyways but we'll be addressing just exactly what is going on here throughout the article. These are the features of this project:
  • Low port micro-ampere current draw
  • Reliable known output source or sink current of 24ma
  • Simple connection
  • Easy construction

OK now I know that doesn't sound like much and it really isn't but that isn't to say that this is trivial or unimportant either. I thought long and hard about the mysteries of the Parallel Port before I built mine and what this board primarily does is remove a lot of the unknowns from the interface. Unknowns such as just how much current can I draw from a Parallel Port? Go out and search I dare you to get a straight answer. Come back when you're as confused as I was and we'll talk about it.

Hook my board up and it is a cut and dried 24ma. If you draw more you're probably only out a 50 cent IC too. That alone HAS to be worth the price of admission! But wait there's more, it slices, it dices, it julians fries ... OK maybe it doesn't do those things. What it does do though is plug into a parallel port and give you convenient buffered connection points.

Like I said they sell these things commercially so it can't be that stupid. It really isn't either.

Step 1: About the Parallel Port

I'm not going to try to educate anyone about the parallel port here. There are a ton of websites all over the Internet about the topic. What I would like to talk about is the parallel port and how it relates to this project though.

I know today's computers no longer come with parallel ports but that isn't necessarily such a bad thing. First off who wants to put a valuable new computer at risk experimenting on it? I know I don't! Second parallel port cards are still available that will plug into any machine with a PCI slot. These add on cards offer several advantages over a built in parallel port as well so they are worth considering if you plan on trying this project for yourself. An add on Parallel Port Card is what I use. USB to parallel port adapters are outside the scope of this project so I won't be addressing those at all. My board will likely work with those as well within the limits of the USB specification. If you don't know what that means search for USB realtime to see if it applies to you.

The advantages of an add on card include the fact that the add on board isn't built into your PC, so there is less risk of damaging the motherboard connecting to one. These cards are far cheaper than motherboards so if you break one you are out a lot less. Also, the add on boards are often more robust than many built in parallel ports are, so a port card is a bit harder to break to begin with. Now if you have an old PC hanging around and you don't care too much if you break it fooling around then using that is fine too. Old PCs can be cheaper than cheap, free even. While add on parallel ports cards are pretty cheap they're not cheaper than free.

The whole point of this project is really not to break anything at all though. The point being that while yes it is fun to play around with stuff, it is less fun to play around with stuff and break it. The hardware discussed in this project is meant to shield and protect a PC from your external tinkering. Now I cannot guarantee that you still won't break something if you try this. But I can say that if you build this project right you will be able to do a bit more than you normally can with just a plain old parallel port.

Step 2: About the Circuit

Let me begin by saying I built my buffer board with a specific project in mind but I'd like to present it here as more of a general application device. The chances of you wanting to do something completely different than I am are pretty good, that being the case you're going to want to do some things a bit differently than I have. You can still take parts of the circuit here and apply them towards your task though.

That is the beauty of do it yourself, that you can customize a project to suit your needs. I don't expect anyone to build their port buffer exactly like I did. Due to parts availability I didn't even build mine the way I wanted to in fact! So I'll present my circuit, discuss a little bit about how it works, then offer some suggestions about ways of going further with it.

My goal here is to give you just enough information that you become dangerous. How you apply it is totally up to you. Here is my schematic, for those of you who have Eagle I have included an sch file as well.

Step 3: Why'd You Make It This Way?

A fair question, like I said I built my board to perform a specific task. Namely to control a CNC machine. But I realize other people have their own things they'd like to do. This circuit is easily adapted and suited for a wide variety of applications as well as demonstrating two separate interfacing techniques. On the data output side it is strictly a buffer, and on the input lines it is optically isolated.

It is really two circuits in one. What you build is entirely up to you, or maybe you'll study my circuit and come up with your own that is better? That is how my circuit came into being. I found someone else on the net who'd made their own but I did not care for some aspects of their circuit so I took the liberty to change things. Really they had no pull up resistors on their inputs, but hey they said it worked for them.

Although I did like their choice of buffer IC they picked to use so I decided to use the same in my circuit. Nothing else between my circuit and theirs is the same other than using the same ICs.

Step 4: About the Parts

I'm looking at my schematic now and I'm noticing I left a lot of values out, and some are just out and out wrong. Most notably the buffer ICs themselves. In order to get the kind of performance I have been talking about 74LS245s cannot be used in this circuit. You have to get the AHCT family IC. So that'd make IC1 and IC3 74AHCT245 s

Why I left values out or just have them plain wrong in my schematic is a combination of reasons ranging from it was easier to do, to the diagram would have just been too busy had I included them. So I'll put a partial parts list here to make up for it.

R1-R12 570
R13 4K7
R17-R20 330
R21-R24 470
RLED1 &2 330
RN1 RN2 100K

Again the 74LS245N is really a 74AHCT245N I also used a different optoisolator but the pinout was the same as the quad opto in the schematic. I used an NTE3221 because I had one lying around. It is a lot of work to make new library elements so I use what is there when I can. That is why the numbers don't always agree. If I can find a device that is close enough physically I use it. You can too.

Here is my Eagle board file.

Step 5: Construction

I built my circuit on perfboard. I suppose if someone is all setup to do it they could etch a board but for this I don't even think it is worth the trouble of drilling all those holes. I laid the parts out so it was easy enough to get everything connected together. The worst part about it all was getting all the wires off the sub D shell connector that plugs into a parallel port cable hooked up.

I of course got my 25 pin sub D and ribbon assembly out of an old computer where it was a port riser. A pair of scissors changed that fast. Who doesn't have a pile of those old things lying around? Well here is a use for one. Connecting the ribbon cable wires to the right places is probably the most challenging aspect of this project and where I expect most people to have problems and or questions about.

There are 25 pins and they all look pretty much alike so this I feel is where the great mystery of the parallel port really lies. It is also where I anticipate the most troubles will arise with this project. The P1 through P17 connection points on my schematic correspond to pin assignments of the parallel port connector. I suppose a secondary feature of my buffer board is to rationalize the connection of the port itself somewhat, although not entirely. I went with whatever laid out the easiest.

This is the order of my buffer board's barrier terminal strip:

1 2 3 4 5 6 7 8 G G G 9 14 16 17 10 11 12 13 Power to board

In an imperfect world I guess my board fits right in. Feel free to change the order of your connections to best suit you.

In the image I have grayed out the background to highlight the 25 wire ribbon cable that comes off the sub D connector connected to a parallel port cable. It is connected to my prototype circuit of this project on a breadboard. That's it, one IC with a couple resistors. This project merely duplicates that simple circuit segment over and over.

Step 6: Construction Addendum

This is so obvious to me that I failed to mention it but that might have been an oversight on my part so I'm mentioning it. When you are making this project construct it in logical stages. Like don't solder it all up, fire it up, then watch the tops of the ICs blow off because your power supply sections weren't right or something silly like that! Build and test the power supply sections first. Larger electronics circuits are constructed of modular blocks that together to form bigger systems.

I find it best to operate on the theory that any complex task can be made easier if it is broken down into more manageable steps. It works for me! So I am putting it out there in hopes it works for others as well.

Some of my Image Notes did not show up and I cannot fix it I am sorry.

Step 7: OK I Made One Now What?

This is the page where I get to totally cop out and say something completely meaningless like, now that you've made this the circuit possibilities are virtually endless! You know, it is sort of fun to say that. All by itself I'll admit this board isn't totally amazing. It is an important link in a chain though. The longest journey begins with a single step and this is that vital first step.

If the parallel port is truly a port then this is your ship that you can set sail in across uncharted waters. Where you go from here I do not know, probably because you haven't confided in me any of your evil genius plans yet. Feel free to make liberal use of the comments section this website so conveniently provides to share your thoughts and we'll write this last section together.

This is what I made mine to connect to:

Step 8: Epilogue Conclusion

This project was essentially over in step 5 but it is really only the beginning of so many other possibilities that I guess I just can't let it go. The major limiting factor of this device is its current handling capability. 24 milli-amperes while substantial in the world of microelectronics still isn't a large amount when it comes to industrial control. Even to energize a relay like the one depicted in the image on step 7 of this article my circuit would still need its output boosted.

While it wasn't a good fit for me I still believe that in a wide variety of applications the ULN2003 Darlington Arrays are a valid choice if the low current capability of my circuit is an issue with your design. Although I also do not see the harm in building my circuit and gaining some experience with the whole interfacing process before moving on to a more ambitious project. My circuit could even come in handy during the prototyping phase of a more powerful circuit.

My circuit could offer some headroom and leeway while a design was worked out and finalized. It is obvious that by itself my circuit is of no practical use. But that is not to say that it is worthless either.

Now this is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning.  --Winston Churchill



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    25 Discussions


    Reply 2 years ago

    What is an RFD component?


    Reply 5 years ago on Introduction

    Nothing. Perhaps some software needs it but not what I use.


    5 years ago on Step 6

    shoot brother! With me seemingly fallen off the water melon truck yesterday and all, I'm having a lot of trouble figuring just what parts I would need to pull one of these together. Learning eagle is a tasking all by it self any change I can just get a close up of your boards (front and back) and a parts list?

    1 reply

    Reply 5 years ago on Step 6

    Schematics are always better to build by. A schematic shows every single connection, a picture of a circuit may not. This is a picture of the top of that board:

    There isn't a whole lot of special stuff going on on the bottom of the board that would help you out any I'm afraid. This is a screen capture of the Eagle trace pattern:

    I actually use that part of the program to help me put boards together, even if I am assembling them point to point. Learning some design software is worthwhile today in my opinion. I need to learn PCB myself. It sort of bugs me that I use Eagle as opposed to a GPL solution. PCB is very capable, so far I'm not very capable using PCB yet myself though.

    I know what the board is on the background of that page though. I've chatted with the person who made it a few times. He is also one of the developers of pcb. That is his BLDC motor driver. It is pretty awesome! Well a little bit of it, this is a picture of the whole board:

    He thought it was funny when I mentioned I'd noticed it being used as the background graphic on PCB's home page.


    6 years ago on Introduction

    Sorry for the noob question, but what voltage caps should be used with this project. I am brand new to the electronics scene, and am going to try this BOB and your stepper motor controller. I will be etching the board, and need to make changes for your component corrections. I don't have the background for this, but am currently reading any information I can to understand. Think this is really cool. I am good with a multimeter and soldering so I am hoping all goes well. Was pretty successful with making a DVA adapter for checking coils on outboard engines, but it was a really simple circuit. Thanks for any help you can provide. I will give pictures if you are really interested in the progress.

    9 replies

    Reply 6 years ago on Introduction

    My circuits are nice and all but there are better solutions available today. Cheaper, easier, all the things that make one decision better than another.

    I will say this right here and now though. Your choice of power supply will dictate your overall project success. A bad supply will leave you with a poorly performing, or even a totally non-functional mess.

    So if you want my advice then begin at the beginning. This is where it gets tricky though because it isn't crystal clear where you have to start. Often a system is like a maze where one parameter dictates earlier decision branches. So it makes sense to work backwards from what you want to where you need to begin.

    Right now I know you want to run stepper motor drivers based on TB6560s. this tells me you need a specific voltage, and some multiple of current. the magic number with TB6560s seems to be about 28V don't let anyone, not even Toshiba snow you with that 32V maximum value. That doesn't take inductive feedback into account at all! Personally I run my drivers at 24V. They work great there too.

    As far as your current requirements go that depends on your stepper motor, or motors you plan on running. I'll use a bit of my system as an example. Lets say I have 3 stepper motors that need 2 amps each. 3 times 2 equals 6. Easy math.

    Now I know I need a 24V supply @ 6 amps. Just bear in mind one spike to 33 volts and it is game over. It could be for less than the blink of an eye but the IC will see it as a shot to the heart. Poof! I know it sounds funny up until it is your board smoking in front of you.

    I'm interested in hearing you're happy like a kid on Christmas day. If you want to get there then get yourself the power supply you need. I can help you setup one of those import driver/interface boards right too. Last I heard a 3 axis board was going for $22. My circuits can't compete with that!

    Though the right current sense resistors could potentially set you back another almost $20 Now you know why the boards may not ship with them :) I hear with some suppliers you can specify what you want, so keep that in mind. That, and a couple of other settings and you're off to the races!

    My power supply.


    Reply 6 years ago on Introduction

    You have given me much to think about. Fortunately I am doing this for the learning experience, and I believe that any knowledge gained is priceless. So the price of a $22 board does not make me jump at buying it; I would rather spend the money and gain the knowledge. Your explanation is wonderfully written by the way. I believe that I might just build the power supply in your instructable and stick with your BOB and driver, I believe I will learn more by doing so. Plus one of the best ways to learn is break stuff. I appreciate your feedback and welcome it. I plan on beating you down with questions though; if you don't mind. I'm in no hurry so I won't bombard you to much. Thanks for your help thus far.


    Reply 6 years ago on Introduction

    My setup does work. Which is more than some of the $22 board folks can say. I went with a linear power supply because PWM motor drivers are pretty noisy all by themselves. I think half the people who have troubles with their stepper setups have them because they're using messed up switching supplies. There are reasons some of that stuff lands on the surplus market I suppose.

    My driver, BOB, and power supply all do work together. I also bought a PCI parallel port card with a Netmos 9815 chip on it. My BOB will work on a regular built in port too but when I was making it I was a little worried about blowing a system up. I forget all of the gory details now but my BOB only draws a fraction of a milliamp from the parallel port on any given data line.

    I worry about breaking stuff myself so I'm pretty careful. I've had some stuff not work for a bit until I figured it out, but so far nothing has broken on me yet. If you look closely at the breadboard prototype of my motor driver you can see I even made a copper scatter shield for the driver IC. I heard the chips had a bad habit of blowing up on a lot of people.

    I only ever made one of those scatter shields. It has never come into play yet for me. I still have all of my spare TB6560s I bought hanging around. I figured I'd probably blow up a few making my driver. I didn't fry one.


    Reply 6 years ago on Introduction

    I'm definately building them. I have a couple of laptops with built in parallel ports that I will try. If I have any issues with the first one I will go with a PCI parallel port card for sure. I am going to start with the power supply first and keep you updated with my progress, once again thinks for the expertise. As the system design progresses I will post every think I am doing. Thanks a million, I now have to start sourcing my components.


    Reply 6 years ago on Introduction

    Before you get too attached to any laptop for your project I suggest you check its latency. Your worst case latency is your base period, which is the heartbeat of your entire CNC system. Laptops by their nature usually have horrible latency figures.

    System latency is unlike any other benchmark you're likely to be familiar with, but the only one that really matters when it comes to CNC. This is one of the pitfalls no one sees coming so I figured I'd warn you.

    But let me know how you make out with that because it is nothing I've any firsthand experience with. I just avoided it all myself. For comparison my trash bin 1GHz P3 has a maximum latency of 23500ns An Intel atom board usually can score about 5400ns which is great. When I'm looking for some more performance it is one avenue I'll pursue.

    BTW wise move to make the power supply first. It is what the rest runs on. I had a heck of a time locating a transformer that put out the right voltage and had the current handling capacity I wanted.

    If you get really pressed rewinding a microwave oven transformer is a possible option. If you're rich just get a nice toroid. I used a combination of clever and luck myself. Turned out I had the right transformer hanging around all the time, I just had to use it funny. I ganged up two 10 volt windings in series of a transformer I pulled out of a PDP 11/34 minicomputer. Which got me a 20 VAC transformer. Rectified and filtered that took me up to about 26VDC, regulated dropped it to 24VDC Close enough.

    I've never run any of my TB6560s over 24VDC I hear they do bad things when you over volt them. Someday I'll have to find out for myself I guess. I have seen some pictures of blown chips. Personally I doubt they could withstand much more than 28VDC input no matter what Toshiba says. If you read closely they never say 32VDC input they say total system voltage or something to that effect. That includes any inductive feedback from your stepper motors.

    So anything past 24VDC input and you're on your own. Its no place I've been and nothing Toshiba is really promising either. This seems to be where lots of hard feelings start to me. Tread lightly.


    Reply 6 years ago on Introduction

    I still have to check the latency of the laptops parallel port, but I figured I would let you know that I have started ordering the parts of the power supply. I figured I would focus on one project at a time, and it seems logical to start with the PSU first.Thanks for all the advice so far. Don't know what I going to use for the transformer yet either. Will keep you posted as I get time to work on this, unfortunately also building cabinets at the same time for mamma. I bet you can guess which one takes priority. LOL.


    Reply 6 years ago on Introduction

    It is the entire system's latency time that matters. But you'll see when you get that far.

    That transformer on ebay looks like it'd do the job to me. You're going to get more DC out of it than its AC rating after rectification and filtering. I'm guessing you should get about 32 VDC out of it or better. Anything under 40 and you're good. Recently going through a bunch of "24VAC" transformers I found most actually put out around 28 VAC. I guess transformer manufacturers don't want to short change people with 115VAC line voltage? Precision transformers have multiple taps to adjust for that sort of thing.

    Be smart and use filter caps about double that voltage. I'm thinking 80 VDC caps would work well. Anything from 50,000-150,000uF should do the trick. You can parallel caps to add their uF. I used one big cap because I happened to have it.

    For big bridges I prefer devices like this:$8195044.jpg

    I'm sure other things work too, but I find the big blocks easy to work with.

    Lots of folks just run with that. A transformer, rectifier, and filter cap. But then your voltage you get out of it has to be spot on. Building the rest of the adjustable regulated supply gives you some margin to play with. Sometimes a volt one way or another can matter. Sometimes it doesn't.

    But that is why some shell out the big bucks for custom wound toroids for this application. If you have unlimited financial resources there is a lot to be said for that method. Some that can be said against it too. I think most who go that route do so because they have more cash than electronic skill. I also wonder how many close but no cigar transformers they store on their shelves, next to their over volted fried motor driver collection of course.

    Then there's the switching regulator crap shooters. Sometimes they win, sometimes they lose ... I laugh when they lose because they never really know why. Though they have their theories, all wrong of course.


    Reply 6 years ago on Introduction

    I also have a RTP1P5C030-T transformer out of an old audio receiver but I can't find any data sheets on it.


    Reply 6 years ago on Introduction
    Would this be a suitable transformer?

    Dream Dragon

    7 years ago on Introduction

    A really useful and interesting project. You mentioned "opto-isolating" your INPUT lines, but shouldn't you isolate the outputs as well to protect against the failure of the controlled device?

    3 replies
    pfred2Dream Dragon

    Reply 7 years ago on Introduction

    For my particular application the short answer is no. The explanation is a bit lengthy as to exactly why. Put simply I've already isolation at the devices and optocouplers are huge speed liabilities that add up every time you pass data through one.

    I will use my system to illustrate and hopefully you shall agree with my design decision.
    • Signal timing <5ms
    • TLP521 optocoupler speed >3ms
    • Most critical gain to be had isolating signal. Stopping noise produced by the motor driver from propagating throughout the system.

    Really I'm just doing the best I can in a bad situation. Adding another optocoupler would only make it twice as bad without gaining me anything of value. I hope this has helped.

    panic modepfred2

    Reply 7 years ago on Introduction

    opto couplers (like all devices) do add delay but it is very small. according to datasheet for TLP521 delay is 3 micro seconds (not milliseconds!).

    pfred2panic mode

    Reply 7 years ago on Introduction

    Thank you for the correction. Switch ms for us in my post. In my haste I used the wrong abbreviation. In the world of digital electronics 3 MICROSECONDS is still thousands of times slower than logic buffers which are some nanoseconds. I hesitate to throw out a figure for fear of further correction. But if I had to guess because I can't be bothered looking the EXACT number up right now I'd say somewhere in the range of 15-20ns. My circuit is running on a 5000 nanosecond clock which is 5 MICROSECONDS. So my point still stands that the delay of the optocouplers is a substantially limiting factor in my circuit application!