Patent GB-1352055-A (GB)
Title: Data communication systems
IPC: H04L5/02
Filed: 1971-04-26
Abstract: 1352055 Communications systems HONEYWELL INFORMATION SYSTEMS Inc 26 April 1971 [29 June 1970] 11454/71 Heading G4A In a data communication system, in response to a character being received from one of a plurality of sources a base address word associated with the source is retrieved from memory and modified by the received character to derive the address of a unique control character stored in the memory, the control character being retrieved from memory and used to process the received character. As described when a complete character is received from a terminal device 60-6n (Fig. 1) by an associated sub channel 7a-7n continuously scanned by a controller 5 the controller stops on the sub channel and an 18 bit base address word is read from an associated location in a memory 3, under the control of address encoder 17 and counter 10 (Figs. 8a, 8b) controlling the scanning of the terminals into a data register 20. It is subsequently read into a register 23, after bits 9 and 10 have been combined with bits 6 and 7 of the received character. Bits 1-7 of the received character are stored in 6 bit register 24, bits 6 and 7 being combined. Bits 0-8 of the base address word represent a base address, bits 9, 10 a modifier field and bits 15-17 a tag field. The base address word and received character are combined in register 31 to form a 14 bit address for the control character. Bits 0-8 of the address word cause selection of one of eight character control tables from N groups of tables. Bits 9- 13 derived from bits 3-7 of the received character modified by the modifying bits 9, 10 of the base address word cause selection of one of 32 words in the table, the two least significant bits of the received character being used to select one of four bytes in the selected word. Bit 11 of the base address word is used to perform a special function. When it is not zero, bits 6, 7 of the received character are examined and if they are both zero a control character is received from memory. If either of the bits are one the control character cycle is inhibited by the generation of a signal IHBCCW and the controller returns to its scan cycle, the received character being stored in the memory of the data communication system without the need for retrieving a control character. This is effected by using an indirect control word for addressing, the address being a section of the store associated with the sub channel providing the character and stored in an associated register 47a-47n from which it is fed via address encoder 17 and gate 36 to the memory. When the signal IHBCCW is not generated gates 29, 32 are enabled to address the store and read a control character into register 20. The control character is of 9 bits, bits 0-2 representing a tag field (the tag of the stored base address word being modified to have the same tag field if it originally differs by comparing the output of tag register 41 holding the tag bits of the control character, these being fed to the memory to modify the stored base address word when a comparator 39 indicates that the tag bits of the base address word and control character differ). Bits 6-8 represent one of eight possible commands. If there is no parity error decoder 43 decodes the command bits and stores each command in the appropriate section of the register 47a-47n. Code 000 indicates that the received character is a normal character to be stored in core memory in the address block set aside for incoming messages from the associated source, the starting address of the block being incremented and the length of the address block being decremented after each received character is stored. A programme interrupt is generated when the length decreases to zero. Code 110 represents a character such as a space, synch or delete which is not to be stored. Code 011 represents a final character and results in a special status word being additionally stored. A programme interrupt cycle is then initiated by the controller to process the message. Other codes 001, 010 represent an end of message followed by, e.g. a block check or parity bit. Code 111 is used when a programme is to be interrupted when a specific character occurs, code 100 being used merely when it is desired to know whether a specific character has occurred. Bit 3 of the control character is a resync bit used when synchronous data transmission is employed between the controller and the terminal to provide automatic initiation of a search mode for recognizing a synch pattern. Bit 4 is a buffer switch bit by which data can be stored in core memory in two buffers used alternately. Bit 5 is a parity inhibit bit used when the data does not include a parity bit, all 8 bits of a received character being used for data, e.g. to transmit a two digit binary coded decimal number. Detection of sequences of two or more characters defining an end of message is effected by "table switching", that is modifying by the tag field bits of the control character the bits of the address word controlling the selection of one of the eight character control tables in each group. If for example an end of text character ETX is followed by an end of transmission character EOT and a block check BCC, the character ETX results in a tag field of 001 being read from the first table and used to modify the stored base address word. Consequently when the character EOT is received the modified base address word is read from the store and the second table is accessed. A command field of 001 is read out from the resulting control character which instructs the controller to store the block check character and then interrupts the programme. Transparent messages are recognized by the sequence of characters data link escape DLE and start of text STX which result in first table 2 of the selected memory section being accessed for the control character, all the control characters in this table having the priority inhibit bit set and a tag field of 2 except for the character DLE which has a tag field of 3. Consequently when the character DLE is received table 3 is accessed, all the tag fields in the table being 2 except for the control character accessed by the character ETX which has a tag field of zero so that when the end of text character is received the system returns to its normal operation.