Patent GB-1513827-A (GB)
Title: Computer interface system
IPC: G06F15/17
Filed: 1975-07-29
Abstract: 1513827 Controlling data transfer between asynchronous computers NATIONAL AERONAUTICS & SPACE ADMINISTRATION 29 July 1975 [5 Aug 1974] 31753/75 Heading G4A  Data transfer between asynchronous computers (A, B), figure 1 (not shown) is controlled by means of a sequence of control signals interchanged between respective interface logic circuits A, B, each received control signal being verified over a predetermined number, e.g. two, of clock pulses of the computer which received that control signal, there being means at the computer intended to receive data which initiates a data transfer to that computer in response to verification of all the control signals of the sequence. In figure 2, which illustrates data transfer from computer (A) to computer (B), the flow of the control signals is shown as a heavy line. Computer (A) first issues an outbound request (OB REQ) signal on a line 15A which is repeated by circuit A as an inbound request signal to circuit B, the inbound request signal being applied to the mode control terminal of a bidirectional shift register 14B to cause it to left shift, entering 1's into register 14B. After two clock B pulses a 1 is present in stage C of register 14B if the inbound request signal is verified, this 1 enabling a gate 18B which provides a return-response signal on a line 19 to circuit A. At the same time a gate 10B is inhibited to prevent computer (B) signalling computer (A) on a line 15B that data should be transferred from computer (B). The return-response signal received at circuit A is fed to a shift-register 28A, where, after two clock A pulses, a 1 is present in stage B of register 28A if the return-response signal is verified. The contents of stage B are fed to a gate 32A which transmits a ready signal to circuit B. The ready signal received at circuit B is fed to a shift register 28B, where, after two clock B pulses, the contents of stages A, B, C are respectively 1, 1, 0 if the ready signal is verified. These states of stages A, B, C, in conjunction with a low signal on OB REQ line 15B, enable a gate 60B to provide a data strobe pulse to computer (B) to initiate data transfer thereto. Although the contents of stages A, B, C of register 28A were also respectively 1, 1, 0. a data strobe pulse was not produced by circuit A since OB REQ line 15A was high. The data strobe pulse is terminated after one more shift of register 28B since the contents of stage C are then 1. Termination of the data strobe pulse causes, via a gate 20B, a termination of the return-response signal to circuit A; this in turn terminates the ready signal. If the inbound request signal is still present, termination of the ready signal will cause the production of another return-response signal. This will cause a further ready signal and data strobe pulse to be produced, resulting in the transmission of the next data byte to computer (B). This process continues until the last data byte has been transferred, when the inbound request signal is removed, and the return-response line is turned off and remains off. Once the formation of a data strobe pulse has been initiated in register 28B, a noise transient on the ready signal (of either polarity) of duration less than two clock B periods will not affect the completion of the data strobe pulse. For example, suppose that 1's are being shifted into register 28B and that the states of stages A, B, C are respectively 1, 1, 1, with a noise transient 0 present at the input of register 28B. After two clock pulses the states of stages A, B, C would be 1, 1, 0, resulting in the generation of an erroneous data strobe pulse; however previous to the first clock pulse when the states of stages A, B, C are respectively 0, 1, 1 this condition is recognized by gates 52B, 54B and a 1 written into stage B on the first clock pulse to overwrite the 0 (noise) which would otherwise be written into stage B. Register 28A operates in a similar fashion with respect to the return-response signal, noise transients of duration less than two clock A periods being ignored.