J. David Irwin The industrial electronics handbook, CRC Press, 1997 ISBN 0-8493-8343-9, Chapter 19 /wiki/ISBN_(identifier) ↩
M. A. Laughton and D. J. Warne Electrical Engineers Reference Book 16th Edition, Newnes, London 2003 ISBN 0-7506-4637-3, Chapter 16 "Programmable Controllers" page 44 /wiki/ISBN_(identifier) ↩