We've thought about doing this on a current project for ESD "protection". With a 15kV discharge the opto was breaking down & took the driver & receiver circuit with it. We think we've fixed the problem using other means but this sort of tracking was seroiusly considered for a while. RP On 21/09/2007, Geo wrote: > On 20 Sep 2007, at 22:53, Xiaofan Chen wrote: > > > Yesterday an engineer showed me a strange thing. It is an isolated DC > > input I/O with relay output. The isolation specification is low. So > > every input is going through an opto-coupler and the circuits are > > quite simple. Strange thing is that there is a long trace underneath > > all the optos and it is connecting to the common shield connection. > > Apparently this is not for safety reasons as it actually reduce > > clearance/creepage distance (still enough). > > I would imagine that is a flashover guard. Spurious high voltage on > an input pin should arc to the protective guard before reaching the > logic side. > > > George Smith > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist