Pretranslator

A pretranslator is a frame in a No. 5 Crossbar telephone switch that is responsible for translating the first 1, 2, or 3 digits of a dialed number. It informs the originating register how many digits to expect in total for a particular call. Pretranslators are called in by originating registers once per call. Their holding time is very short--only a fraction of a second, so two pretranslators is usually sufficient for all but the largest offices.

Rationale and Technical Info

Unlike other types of crossbar telephone switches, the 5XB originating register normally requests a completing marker only after all 7 or 10 digits of the called number have been dialed. (A No. 1 Crossbar, for example, calls in a marker after the first three digits have been dialed.) Because of this, handling of calls with less than the expected total number of digits is a bit more complicated. For example, if the originating register (OR) always expects 7 digits, and the customer dials a three-digit service code, such as 611, the register would normally wait for four more digits to be dialed before attempting to complete the call. Thus, the subscriber would have to dial 611-1111. The reverse is also true. If some calls require a full 10 digits, and others require only 7, then the OR needs a way to discriminate between them, and call in a completing marker after the desired number of digits have been dialed, rather than waiting unnecessarily.

To accomplish this, the OR calls in a pretranslator after the first 1, 2, or 3 digits are dialed (as necessary). The pretranslator translates the dialed digits, and returns instructions to the OR, informing it how many digits to expect in total. If only three digits are to be expected, the OR will then call in a completing marker immediately. If, however, a full 7 digits are to be expected, the OR will release the pretranslator, and await four more digits before calling in a completing marker.