Patent GB-1385061-A (GB)
Title: Communication system between a central computer and data terminals
IPC: H04L12/00
Filed: 1972-01-26
Abstract: 1385061 Data processing system ING C OLIVETTI & C SpA 26 Jan 1972 [1 Feb 1971 11 June 1971] 3731/72 Heading G4 [Also in Division H4] A data processing system includes a central computer, a group of data terminals connected to the computer through a transmission channel, a line controller, and a common concentrator, each terminal being able to signal a condition of readiness to transmit, the concentrator and line controller being operable by the computer to control the despatch of data to the computer, and the concentrator including common communication means between the terminals and the line controller, and connecting means controlled by the terminals for connecting a terminal which is ready to transmit directly to the common communication means. General.-Three modes of operation are described viz. simple polling where the terminals are scanned in a fixed sequence to determine whether any have data to transmit, addressed polling where a particular terminal is addressed to determine whether it has data to transmit, and selection where a particular terminal is addressed to receive data from the computer. Addressing is by means of two eight bit characters the first of which addresses a concentrator. The second character may specify (for selection) in bits 1-4 the address of a terminal, and in bits 5 and 6 which mode of operation is required. The system may also include a number of other terminals each connected to the computer via a respective line controller and transmission channel. The arrangement is such that a selected terminal of a group is connected through its concentrator directly with its line controller so that the data format used by the computer is identical for both groups of terminals and individually connected terminals except that in the latter case only one address character is required.  If a concentrator does not recognize its address signal RI on line 30 is zero. Thus OR gate 23 supplies zero so that flip-flop 24 remains set, its output X=0. Switch 14 provides an output U= input A when X=0 and U= input B when X=1. Thus the state of counter 10, staticized in register 11, is connected to gates 32 which are opened by signals B indicating that no terminal is ready to transmit. The appropriate terminal recognizes the four-bit address and if ready to transmit produces a signal NP=0 and A=1. The system remains in this state until the concentrator recognizes its address (see simple polling below). If however the terminal is not ready to transmit it produces NP = 1 to increment the counter and scan the next terminal. Simple polling.-When the concentrator recognizes its address RI= 1 is generated. For simple polling bits 5 and 6 of address character two are 0 so X remains at 0 and gate 34 is blocked so that bits 1-4 are ignored, and the terminal selected by counter 10 (see above) is connected to the line controller via gates 32, 42 since the appropriate signal B=1. Addressed polling.-In this mode bits 5 and 6 are 1 and 0 respectively so gate 34 is opened to pass bits 1-4 to register 11. OR 23 sets flip-flop 24 to produce X=1 so that bits 1-4 also pass via line 15, switch 14 and gates 32, 42 which are opened by signal PIS generated when addressed polling or selection are required, to the terminals. Selection.-Bits 5 and 6 are both 1 so that gate 34 is inhibited. Gates 25 and 41 are however conductive so that bits 1-4 on line 15 pass to the terminals and into register 39. Gates 32 and 42 are again opened by PIS so that the terminals receive the address bits. Data transmission.-When, in response to simple or addressed polling, a terminal responds to its address and assuming it is ready to transmit it generates a start signal TCl which closes gate 47 and opens gate 50 so that its address from register 11 is transmitted via line 12 and counter 44 which adds a parity bit. Gate 51 is also opened so that an eight bit character is made up by three fixed bits from register 54. The data is then transmitted. When a terminal responds to "selection" the processor prefaces its data with a repeat of the address characters. Bits 1-4 of the second character are compared with the contents of register 39 to give an error check. Further details.-Parallel-serial and serialparallel converters may be provided between the concentrator and the terminals. The converters may comprise clocked shift registers.