Patent GB-1159183-A (GB)
Title: Pulse Signal Exchange
IPC: H04L12/54
Filed: 1967-05-26
Abstract: 1,159,183. Computer input/output. INTERNATIONAL STANDARD ELECTRIC CORP. 26 May, 1967 [31 May, 1966], No. 24583/67. Heading G4A. Data signal exchange apparatus scans signalling lines and corresponding memory cells in turn, a signal element being transferred between a line and its cell under partial control of the cell contents, and when a whole character has been so transferred control information in the cell is altered to transfer a character between the cell and a word store for holding a group of characters, and group of characters can be transferred between the word store and a processor. In synchronism with scanning of input and output transmission lines, a status word and a data word are obtained from a core store in turn corresponding to each line. In case of low speed input lines, a serial by bit character arriving on the input line is assembled in a onecharacter portion of the status word, up to one bit being sampled and inserted during each scan cycle. Sampling occurs whenever a scan count portion of the status word, preset in accordance with line speed as specified by further bits in the status word and decremented by one each scan cycle, reaches zero. High speed input lines do not use a scan count. A completed character in the status word is transferred (under control of control information in the status word altered in response to the completion), with or without code conversion from Baudot to ASCII as specified by a bit in the status word, and via an exchange register, to one of four character portions of the data word. When a pair of characters are present in the data word, they are sent via a receive register to a computer when the latter can be interrupted, an accompanying tag identifying the source transmission line. Data transfer from the computer to an output line is the reverse of the above, a send register replacing the receive register. When a two character section of a data word has emptied, a request flip-flop corresponding to the output line involved is set. The states of the request flip-flops are sent to the computer via the receive register with interrupt to obtain more pairs of characters. In the case of some lines, message delimiter characters are tested for (though in some cases only if a certain bit of the status word is set) detection causing the current character pair to be completed, if not already completed by the delimiter character. Sync signals accompany data on transmission lines. Error features.-Each status word contains a two-bit security tag set by the computer. In the case of input lines, this tag is copied into the data word as characters are transferred from the status word to the data word, and is sent to the computer with the character pairs to enable a check to be made as the computer sorts the pairs into per-line blocks. In the case of output lines, each character pair from the computer is accompanied by the security tag which is placed in the data word. Whenever a character transfer from data word to status word is attempted, a comparison of the security tags in the two words is made. If they are unequal, a blank is sent to the status word and the computer is notified by setting a monitor bit in the status word which is then detected by the computer responding to a further bit set in the bit group sent to it to obtain further character pairs. The further bit does not identify the line involved whereas the monitor bit does. Similar notification of the computer occurs in the case of overflow, viz. when the character spaces in the status and data words for a given line are all full.