This content-rich site for all things related to instrumentation contains, among many other links and references, a free downloadable copy of Programmable Controllers, Theory and Implementation, 2nd ed. by L. A. and E. A. Bryan. Over 1000 pages of controller information. It's a 5.30 MB download, but worth the time. The direct link is at http://tinyurl.com/ycxco6k
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