Abstract: The isochronism of binary counters can be checked by comparing the counting positions. If in a system two counters are required which are operated isochronously but not in synchronism, for example the individual read and write counters for memory addressing purposes, the arrangement according to the invention provides a simple solution by using one counter as a duplicate of the other one instead of duplicating both counters. The parity of the counting position after each increment of both counters is generated. The parity bit of the counter having the highest counting position is delayed by means of a shift register over a number of positions corresponding to the difference in counting positions between the two counters and is then compared with the parity of the counting position of the other counter.

Abstract: To establish a conference connection between three participants in a TDM telephony system, use is made of the time-division switching stage which in many cases is already of a duplicated construction. The information signals (a, b, c) from the participants are written into corresponding addresses (AA, BB, CC) of two information memories while these addresses, after having been cyclically permuted, are written into the address memory associated with each of the information memories. When now the locations in the information memories are read in the sequence indicated by the address memories and summed thereafter, then each participant in the conference is supplied with a conference signal which is equal to the sum of the information signals of the two other participants in the conference connection.