Hans Thanks for the WW reference. I must admit I had got the PE Digical calculator confused with another WW article also by B.Crank. Can't remember the title but it was a very simple logic analyzer using a scope as a hex display. I remember it used Ferranti DTL chips. Also remember reading the PE Digical calculator articles - I can still picture the calculator - a four function desk top machine full of TTL ! Chris Leyson