A 38 Page pdf available at http://www.samhallas.co.uk/repository/documents/pentex_exchange.pdf
Electronic Telephone Exchanges: TXE2-A Small Electronic Exchange System Page 12-20
Can be obtained from
http://www.samhallas.co.uk/repository/journals/POEEJ/POEEJ%20Vol%2062%20Pt%201%20April%201969.pdf