Encoding switches

Abstract

Where switches are used for encoding decimal digits into binary words for use in processing devices in, for example, postal franking machines where security is essential, failure modes that produce normal codes not corresponding to the input decimal digits have to be detected. The switch described reduces the number of failure modes which the processor cannot recognize as errors in that it produces two 1-out-of-5 coded words for each decimal digit to enable a processing device to test whether each word in fact contains only one '1' (or '0') and then to combine the two words to produce a 2-out-of-5 word corresponding to the selected decimal digit. The switch comprises five contact elements A to E each comprising inner and outer arcuate portions 10, 11 concentric with common rings 13, 14. A rotatable part having ten positions 0 to 9 can be set in any one of those positions to select a decimal digit, and comprises connected contacts for bridging an inner arcuate portion 10 and ring 13 as well as, at a diametrically opposite point, an outer arcuate portion 11 and ring 14. When the rotatable part has been set, the two corresponding 1-out-of-5 codes are signalled by scanning in sequence the common rings and reading the five contact elements A to E.