>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 19 October 2005 20:42
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE]: Could PC Parallel port drive a device that 
>needs TTLtriggers?
>
>
>On Wed, 2005-10-19 at 11:45 -0700, John Waters wrote:
>> Hi All,
>> 
>> I have a equipment whose operation invloves being triggered by a 
>> continuous
>> pulse train of 1 Hz at TTL level. Currently I'm using a 
>funtion generator 
>> with TTL o/p for doing that, but I want the operation be 
>more automatic, so 
>> I'm considering using the parallel port of a PC to replace 
>the function 
>> generator. However I wonder if the parallel port of a PC is 
>TTL compatible. 
>> Could anyone give some comments?
>
>Most are TTL compatible. Some will only be 3.3V (i.e. 
>laptops), which might still work fine with TTL hardware, but I 
>wouldn't count on that.
>

Not just laptops either, many modern desktop machines have 3.3volt logic levels on the parallel port.

>A bigger problem is how accurate do you need that 1Hz to be. 
>If you're running just DOS you can get it pretty close, but in 
>Windows expect that 1Hz to glitch every once in a while.

Indeed.  A 555 timer is likely to offer far better short term stability and costs pennies.  A 32.768kHz watch crystal and binary divider costs litrle more, and gives you excellent accuracy and stability.

Regards

Mike

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