You MIGHT consider using RS485/RS422 interface devices and transfer differentially. These chips can handle a GND offset of as much as 7V. Its about as fast as possible for a noisy interface. About $0.50 USD per channel. --Bob > >> -----Original Message----- >> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] >> Sent: 25 September 2006 14:38 >> To: Microcontroller discussion list - Public. >> Subject: Re: [EE] I need a circuit for a bidirectional opto >> isolated TTLinterface >> >> >> On 9/25/06, Michael Rigby-Jones >> wrote: >> >>> >>>> -----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. >>> >> 6N137 it's a common/cheap one (about $0.6 here if you buy >> one): >> http://www.datasheetcatalog.com/datasheets_pdf/6/N/1/3/6N137.sh >> -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist