For some of us (including me!), our first brush with "serial" was 60mA current loop data, a standard that I see you've ignored :-) With some of the loop supplies being able to go up to 50, 75, or even 100V in "open circuit" configuration, I'd *hate* to see what would happen to TTL connected directly! (I have to admit that I got started late enough that optoisolators were the preferred way to go from current loop to other signaling levels... but I do remember polar relays.) -- Tim Shoppa Email: shoppa(a)trailing-edge.com Trailing Edge Technology WWW: http://www.trailing-edge.com/ 7328 Bradley Blvd Voice: 301-767-5917 Bethesda, MD, USA 20817 Fax: 301-767-5927