Keyboard circuit for binary signals in telephony - has matrix with crosspoint switches each comprising earthed contact and antiseries diodes

摘要

The keyboard circuit has columns and rows with keys at the cross-points. The column lines are connected together at one end to a shared rail to which +5V is applied. Likewise, the row lines are connected together at one end to a shared rail to which +5V is applied. Each crosspoint key consists of an earthed contact (I) that applies earth potential to the junction between two antiseries diodes (D1, D2) connected between one column line and one row line. When a contact, is closed a detectable voltage change occurs on its associated lines.