Patent GB-1325194-A (GB)
Title: Electrical signalling system
IPC: H04Q3/545
Filed: 1970-05-27
Abstract: 1325194 Scanning for line states in processor controlled exchanges PLESSEY CO Ltd 19 May 1971 [27 May 1970] 25580/70 Heading H4K A line group is scanned and line states are compared with last look states recorded in a stepping register, any change of state detected during a scan causing read-out of the register provided that the next scan of the group shows no change. If the next scan shows a change in respect of any line the register is not read out to the processor, such read out being delayed until a scan of the line group shows no change. By such means transient changes as may occur with contact bounce are not made the subject of a demand for processor time. In Fig. 1 a group of contacts 1S to 96S are indicative of on-hook, offhook conditions and are scanned sequentially from decoder CD driven by counter PC supplied with stepping pulses on lead SC from a processor CE. For each step of the counter to mark leads t1 to t97 the corresponding line state is registered in the first stage of stepping register SR and is compared with the last look result for that line which has progressed to position 97 of the register. Exclusive-OR gate G5 detects change of state in respect of any line during a scan of the group and sets a bi-stable TA. At the end of the scan, lead t97 is marked to cause the setting of TB as primed by TA which is itself reset but not soon enough to open gate G7 and permit the setting of TC. If the following scan finds no change of state, TA will remain reset and the mark on t97 at the end of the scan sets TC followed by TD to send a demand signal DEM calling for reading out of the register SR. TD set halts the scanning by blocking the stepping pulses SC. The processor sends a prolonged pulse on ACC and a train of 97 pulses on ROC. By this means TB, TC, and TD are reset, the scanning is held back and the pulses on ROC read out SR to line DO while at the same time rewriting the contents over gate G4. When reading out is complete, the pulse on ACC ceases and scanning restarts. If the scan following detection of a change of state in the group also exhibits a change of state, TA will be set at the end of the scan and reading out will not be demanded. TB, however, remains set so that the first subsequent scan to show no change of state will result in reading out. If the processor wishes to inspect the contents of register SR it may effect reading out by a signal INSP which, on scan pulse t97, causes TD to be set direct. Alternatively the processor may operate one of the contacts being scanned to induce reading out in the normal manner. The scanning may be effected co-ordinately, row by row of a 10 x 10 matrix, Fig. 3 not shown. The elements being scanned may be arranged in four groups of 24, Fig. 5 and 6 not shown, each group having its own separate shift register for last-look results, and the scanning counter, which scans the groups in sequence, steers the scan results to each of the shift registers in cyclic sequence. If at the end of the scan of a group of 24 a change of state in the group has been recorded, that group is immediately rescanned to obtain a no-change result before the group register is read out to the processor. Scanning then proceeds over the next group. With each reading out of a group register a code is sent to the processor to identify the group concerned.