1. A remote diagnostics system for POS appliances, wherein said system comprises: a central control unit including a display and a memory; and a plurality of POS appliances remote from and serviced by said control unit; wherein said control unit includes therein: a plurality of problems which may occur on said appliances; a plurality of diagnostic queries which may be answered manually by a operator of the control unit or in an automated manner by said control unit; a plurality of diagnostic rules associated with each of the automated queries, each said rule acting on data obtained from said appliances to provide an answer to its associated query; a decision tree for each of said problems, each said decision tree relating various of said queries with one another in accordance with the possible answers to said queries; and a diagnostics engine which, on input of a said problem, executes a series of said queries in a sequence determined by the decision tree for said problem and by the answers to said queries, said diagnostics engine displaying said queries on said display, as well as prompting said operator to answer queries which require a manual input.