Good day to all. I ran across an interesting and useful paper talks about the problems=20 (and some solutions) related to designing equipment that interfaces=20 to telephone systems. The paper focuses on the requirements for the=20 ringing generator at the telco central office but includes useful=20 information on talk battery connection methods. It also give hints=20 on how to design ring detection circuits at the consumer end of the=20 telco line and mentions specific pitfalls to avoid. The company is Cambridge Electronics Laboratories and is located at=20 . Be warned that the website is fairly ugly and I=20 find it hard to read. Nonetheless, the visual aspects are irrelevant=20 so long as you can find what you are looking for. The paper is titled "Black Magic" with the subtitle "Telecom Design=20 Tricks". The direct download link is=20 . I think that this is a worthwhile addition to the library of anyone=20 who has to work with or interface to telecom equipment. dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .