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How much current from a laptop parallel port output pin?

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JohnJohn20

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I have read that parallel port output pins can deliver anywhere from 2 mA to 20 mA safely.

I read one post that led me to believe that I could take 10 mA to drive a tranistor buffer from my laptop parallel port pins.

Then I read elsewhere that 2 mA was a safe current to take.

Are laptop parallel ports different from PC parallel ports perhaps?

Or are old laptop ports less powerful than the newer ones?

Should a 10 year old laptop like mine be capable of providing 10 mA?

Thanks.
 

HTA

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Hi JohnJohn20,

If your laptop(model?) has a parallel port for a LP-printer then it should support that specification ( http://en.wikipedia.org/wiki/IEEE_1284 ), then it should be defined. Newer laptops/notebooks have a USB-port which is also well specified. What do you want to drive from the laptop?

Enjoy your design work!
 

JohnJohn20

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HTA. Thanks.

I want to drive opto couplers (perhaps 10 mA) and switch some power transistors. This will form the basis of a security alarm which will also be able to manipulate a mobile phone (to send TXT messages) and a camera to take photos when intruders trigger motion detectors.

That specification link yo gave still didn't specify the current capability of these port pins although I did look at one of the refernces that said it can vary from 4 mA to 20 mA.

Do you think my (old) laptop parallel port will have this information specified anywhere?
 

HTA

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Hi JohnJohn,

I think you could assume that your (old) laptop has the standard TTL-specification. If you can identify the type of output driver on your Laptop PCB you could find out if it is TTL, LSTTL or CMOS. If not you could check the output voltage by connecting a defined resistor to ground (output driver set to logic HIGH) or to 5V (output driver set to logic LOW). Standard LS-Driver can drive -0.4mA on High-level and 8mA on Low Level. Buffer could go to -3mA/24mA. Std-CMOS bus buffer could be +/-8mA. If you are not sure it is better to use an interface buffer or some universal I/O for harsh environment (here are some examples for that: http://www.ichaus.biz/wp1_mcu_interface ).

Enjoy your design Work!
 

JohnJohn20

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Hi HTA. Thanks but I am stil not sure.

Assuming it is TTL. what kind of output current can it source and sink?

Would it be safe to just work on 1 mA?

Second. By checking the output voltage as you describe, what exactly am I doing?
- Am I measuring the current from an output pin set HIGH to the point where the output voltage starts to drop?

Or say I connect a HIGH pin to CROUND via a 10K resistor to see if it can source 0.5 mA, and if it can't then I know they are Standard LS-Driver?

Thanks.


and LOW
 

HTA

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Hi JohnJohn20,

Most like likely it is not TTL(due to power consumption). So maybe LS-TTL or CMOS. If it is LS-TTL the typical push-pull output can source (resister on the output to Ground) -0.4mA und sink (resistor from the output to 5V) 8mA. If it is CMOS it should be +/-4mA or more. In this data sheet http://www.nxp.com/documents/other/HCT_USER_GUIDE.pdf you find in table 1 the different technologies and it output drive listed. Fig. 3 of this shows the output stage of CMOS. LSTTL is similar but have bipolar transistors.
In measuring the output voltage under resistor load you check the drive capability. With the clearly define voltage level by technology (see table 1) you get a glue about the output drive capability. Mostly they are better than the specification at room temperature an nominal supply voltage.

Enjoy your design work!
 

JohnJohn20

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OK. I think I got that but just to be sure...

If the chip is LS TTL and I try to source more than 0.4 mA from a HIGH output pin, I will see the voltage between that pin and GND begin to drop from 5.0 V to say 4.5 V. Right?

So how much current can this pin source before it does irrepairable damage to the chip?

In order to protect the parallel port driver chip:
- Could I move data to and from the parallel port via multiplexing chips? This would be useful and if anything went wrong these chips will probably die first.
- Would 74HC42 (TTL BCD to Decimal decoder) and 74LS157 (Quad 2-1 multiplexer) chips be suitable for this?

Could I safely connect the outputs of these chips directly to my parallel port input pins (and vice-versa) or would I need to put series resistors between?

Thanks.
 

HTA

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Hi JohnJohn20,

Your conclusion on the LSTTL output is correct. The data sheet normally state the maximum current on a pin. If you stay below 10mA on one pin at a time, it should not be a problem. Here you find some example of interfaces to the parallel port:
http://www.epanorama.net/circuits/parallel_output.html#circuithow (chapter 2) and http://books.google.de/books?id=hjE...istory&pg=PP1&redir_esc=y#v=onepage&q&f=false on page 252.

Enjoy your design work!

- - - Updated - - -

Hi JohnJohn20,

I just found a complete report on the printer port design/application: http://content.imamu.edu.sa/Scholars/it/VisualBasic/Parallel_Port_Complete_Programming,_Interfacing_and_Using_the_PCs_Parallel_Printer_Port.pdf .

Enjoy your design work!
 

HTA

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Hi JohnJohn20,

I see, there are some problems around Google seach. Maybe just try to Google seach "Parallel_Port_Complete_Programming,_Interfacing_an d_Using_the_PCs_Parallel_Printer_Port.pdf" . You might get the full document.

Enjoy your design work!
 

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