>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 25 September 2006 11:26
>To: 'PICLIST'
>Subject: [EE] I need a circuit for a bidirectional opto 
>isolated TTL interface
>
>
>I was thinking of using HEX inverters and opto isolators that 
>basically are two unidirectional circuits back to back.
>
>Works in simulation but do not know how well it will work in 
>the real world :-(
>
>I want to opto isolate the JTAG programmer interface from the 
>real world as I may otherwise and with Murphy's Law help blow 
>up my programmer :-(

The first problem is likely to be speed related, common/cheap opto-isolators are v..e..r..y  s..l..o..w.

I built a simple opto-isolator for an I2C bus using 4N36 devices and struggled to get much more then 10KHz out of it.  You can persaude a bit of extra bandwidth out of them with a few tricks, see http://www.embedded.com/showArticle.jhtml?articleID=49901764 for details.  However, for typical JTAG speeds I think you may need some fairly exotic/expensive isolators.

Regards

Mike


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