DIGITAL SYSTEM INCLUDING LINE ACTIVITY DETECTION

Abstract: A digital system includes a central system (10, 20) which may be connected via a switching interface (30) including a plurality of ports (31-31'') to peripheral adapter devices (40, 41). An activity detector (300) coupled to each port detects when a data signal is applied to the port, such data signal being transmitted via a multiplexer (320) to the central system (10, 20). If no data signal is detected the multiplexer (320) is switched to transmit to the central system (10, 20) a predetermined, all "1's", signal generated by a ones generator (340). Thus, the reception by the central system (10, 20) of noise which could be generated by a port having no peripheral or a powered-off peripheral attached thereto, is prevented.

Description: DIGITAL SYSTEM INCLUDING LINE ACTIVITY DETECTION Technical Field This invention relates to digital systems of the kind including a central system, a plurality of ports and interface means adapted to selectively interconnect said central system with said ports. Background Art In data processing or computer systems, also denoted herein as a central system, having at least one processor, many methods exist for interfacing the pro¬ cessors of the data processing systems to shared per¬ ipheral devices. The processors may time share a per¬ ipheral on a time basis or the processors may be con¬ nected to a shared peripheral via an adapter, the adapter allocating the shared peripheral to the processors on a request basis. Another recognized architecture for pro¬ cessor communication with a shared peripheral is through the use of a common bus. In these aforementioned system configurations, the sharing of the common peripherals is a so-called dynamic state, i.e., the sharing is varying rapidly under the action of the data processing system. An alternative switching arrangement is a non-dynamic or static state, i.e. where the peripherals are manually switched to a processor by operator intervention. If a processor is switched to a peripheral port which does not have a peripheral or the peripheral is powered off there is a possibility that a port so connected will pick up and amplify noise from a connected cable or open connector. There is a particular risk of this with a manually operated (static) switching interface. Thus, there is a disadvantage that the amplified noise may be sensed by the processor, thereby producing processor errors. Disclosure of the Invention It is an object of the present invention to O PI provide a digital system of the kind specified wherein the aforementioned disadvantage is alleviated. Therefore, according to the present invention, there is provided a digital system of the kind specified, characterized by signal detection means coupled to said plurality of ports and adapted to detect the presence of a data signal applied to any one of said ports, a predetermined signal generator, and switching means adapted, in response to said signal detection means detecting that any one of said ports fails to have said data signal applied thereto, to disconnect the central system from that port and couple said central system to said predetermined signal generator. It will be appreciated that in a digital system according to the invention a port having no data signal being applied thereto is disconnected from the central system and the predetermined signal generator provides a predetermined signal which may be recognized by the central system. Thus the reception by the central sys- tern of noise signals from a port having no data signal applied is inhibited. Where the data signal alternates between first and second states, the signal detection means preferably includes propagation means adapted to receive the data signal and to propagate the second state until propaga¬ tion is terminated in response to receiving the first state. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is an overview of the central system environment in which the present invention finds appli¬ cation; Fig. 2 shows the location of an activity detector within the environment of Fig. 1; Fig. 3 shows a block diagram of the preferred embodiment of the activity detector; Fig. 4 shows a logic diagram of a signal detector of the preferred embodiment of the activity detector; Fig. 5A shows the signal waveforms of the signal detector; Fig. 5B shows the waveforms of the signal detector when the transmission activity ends; and Fig. 6 shows a logic diagram of a ones gener¬ ator of the preferred embodiment of the signal detector. Best Mode for Carrying Out the Invention Referring to Fig. 1, there is shown in a preferred embodiment, a first central system 10 and a second central system 20 (CS1 and CS2 respectively) which are connected to an external static channel exchange (ESCX) 30 through which CS1 and CS2 can access the various peripheral adapters, PA1 , PA2, ... , the peripheral adapters each being connected to a respective port 31, 31', 31' ' of the ESCX 30 by its associated cable 50, 50", 50' '. The ESCX 30 is a switching inter¬ face which provides serial data link communication paths between a central system and a peripheral adapter. CS1 is shown connected to PA1 40. The operation of the ESCX 30 is such that a manual operation is required to con¬ nect CS1 to PA2, PA3,... A central system can also be connected, via the ESCX 30, to the port 31' ' which has no PA connected. Such a condition is depicted by CS2, which is shown connected to the port 31' ' which does not have a PA attached thereto (denoted by the dotted lines of PA3) . As a result, noise pick-up associated with port 31" ' may cause CS2 to accept the noise pick-up as an input transmission. The input transmission is then subject to being checked by the CS2 in accordance with an IOSS (input-output subsystem) protocol which is described in detail in published International patent applications No. WO 82/00373 and WO 82/00374. The input transmission will more than likely result in error indications to the CS2. In order to circumvent these error indications caused by the noise pick-up when no PA is attached to a port, an activity detector is included within the trans¬ mission path or link between the CS and the PA. Refer¬ ring to Fig. 2, the activity detector 300 is shown as an element of the ESCX 30 in the path between the port 31' ' and CS2 20. It is to be understood that the activity detector 300 may be in the transmission link from the CS as well as to the CS. Referring to Fig. 3 there is shown a block diagram of the preferred embodiment of the activity detector 300. The activity detector 300 receives an input signal, the input signal being a serial data input (SDI), which is coupled to a signal detector 310 and a multiplexer MUX 320, the MUX being a 2 x 1 MUX. A clock 330 which generates a clock signal is coupled to the signal detector 310 and to a ones generator 340. The output of the ones generator 340 is a ones signal (ONES) which is coupled to the MUX 320. If a serial data input signal is present at the input of activity detector 300 as determined by the signal detector 310, the output signal from the signal detector (CONTROL) has a logic "one" or high state such that the MUX 320 is conditioned to pass the SDI signal to the output of the MUX 320, the output of the MUX being the signal passed to the central system, or the receive signal (RECEIVE) . If the serial data input signal is not present at the input of ac¬ tivity detector 300 as determined by the signal detector 310, the CONTROL signal from the signal detector 310 is conditioned to a logic "zero" or low state effectively decoupling the input of the activity detector 3.00 from the output of MUX 320. In the preferred embodiment, the CONTROL signal in a low state causes the MUX 320 to be switched such that the ONES signal is outputted from MUX 320, the RECEIVE signal in this case being the ONES signal consistent with the IOSS protocol. Referring to Fig. 4 there is shown a logic diagram of the signal detector 310 of the preferred embodiment. The signal detector 310 of the preferred embodiment is comprised of four toggle or memory-type elements, or more specifically D type flip-flops (F/F) 312, 313, 314, 315. The first flip-flop 312 has its input (D), or D input, adapted to receive the serial data input signal (SDI). The unconditional set input (S) of all flip-flops of the signal detector 310 also have coupled thereto the SDI signal. The clock input (C) of all the flip-flops are coupled to the clock signal. The second, third and fourth flip-flops, 313, 314, 315, have their respective D inputs coupled to the output (Q) of the preceding flip-flop in a chain-like arrangement. The output of the fourth or last flip-flop 315 is the output of the signal detector 310, or the CONTROL signal. The signal detector 310 of the pre- ferred embodiment is comprised of two 10131 high speed dual D flip-flops. The 2 x 1 MUX 320 of the preferred embodiment is a 10159 integrated circuit chip. The operation of the flip-flops of the signal detector 310 can be seen by referring to Fig. 5. Wave- form A shows the bit significance of the SDI signal (waveform B). Waveform C shows the clock signal which is asynchronous to the SDI signal, the frequency of the clock being equal to twice the high frequency of the SDI signal of waveform B. Waveforms D through G show the respective outputs of the flip-flops 312, 313, 314, 315. As indicated above, the output of the fourth flip-flop 315 is the CONTROL signal (waveform G) . When the serial data input signal is high, the flip-flops of the signal detector 310 are set via the S input. This causes the CONTROL signal (the output of the fourth flip-flop, Q4, waveform G) to be in a high state. The CONTROL signal, which is coupled to MUX 320, is forced to remain OMPI high while the SDI signal is high. When the SDI signal is low, the output of F/F 312, Ql, will go low coin¬ cident with a clock signal. However, the serial data input signal must stay low for at least three clock cycles for the CONTROL signal to go low. From waveform B, it can be seen that the nature of the SDI signal is such that the SDI signal changes state (high to low or low to high) at a rate which is lowest for consecutive ones and highest for consecutive zeros. Hence, before the low state of the SDI signal detected by first F/F 312 has a chance of propagating through to the fourth F/F 315, the first F/F 312 will be set, along with all the other F/Fs, causing the propagation action to recom¬ mence. When this type of transmission activity is sensed, the CONTROL signal will always remain high. When the serial data input signal goes low for three clock cycles as shown in Fig. 5B, the input signal passes, in turn, through each of the clocked flip-flops- of the signal detector 310, the CONTROL signal finally going low indicating there is no activity on the attached link. It can be seen by one skilled in the art that the number of F/F stages of the signal detector 310 is a function of the SDI signal frequency with respect to the nature of the noise signal being encountered. Fig. 6 shows a logic diagram of the ones generator 340. The ones generator 340 comprises two D type flip—flops which divide the clock signal by four, producing the ONES signal. In the preferred embodiment, the 32 MHz clock signal input to the ones generator 340 is divided to produce the required 8 MHz ONES signal which is output on the transmission link consistent with the IOSS protocol referred -to above. The flip—flops of the preferred embodiment of the ones generator 340 are 10131 integrated circuit chips. The external static channel exchange ESCX 30 could be replaced by a dynamic-type channel exchange, although it will be appreciated that an undesired switch- ing of a central system to connect with a port having no peripheral or a powered-off peripheral attached thereto is less likely than with manually operated switching.

Claims: CLAIMS : 1. A digital system including a central system (10, 20), a plurality of ports (31-31' ') and interface means (30) adapted to selectively interconnect said central system (10, 20) with said ports (31-31' *), characterized by signal detection means (310) coupled to said plurality of ports (31-31' ') and adapted to detect the presence of a data signal applied to any one of said ports (31—31' '), a predetermined signal gener¬ ator (340), and switching means (320) adapted, in re- sponse to said signal detection means (310) detecting that any one of said ports (31-31' ') fails to have said data signal applied thereto, to disconnect the central system (10, 20) from that port and couple said central system (10, 20) to said predetermined signal generator (340). 2. A digital system according to claim 1, characterized in that said data signal alternates between a first state and a second state and in that said sig¬ nal detection means (310) includes: propagation means (312-315) adapted to receive said data signal and to propagate said second state through said propagation means (312—315), said propagation means (312—315) being adapted to terminate the propagation of said second state in response to receiving said first state, the propa- gation of said first state to a predetermined location in said propagation means conditioning said propagation means (312-315) to provide a control signal for said switching means (320). 3. A digital system according to claim 2, characterized in that said detection means (310) in¬ cludes clock signal generating means (330) and in that said propagation means includes a plurality of serially connected detecting devices (312-315) operative in 3 . ( concluded ) response to said clock signals, the first detecting de¬ vice (312) being adapted to detect said second state of said data signal and the remaining detecting devices (313-315) being adapted to detect the state of the pre- vious detecting device (312-314) in the serial connec¬ tion. 4. A digital system according to claim 3, characterized in that said detecting devices include respective flip-flop circuits (312-315) adapted to be unconditionally set to a given state in response to said propagation means receiving the first state of said data signal. 5. A digital system according to claim 1, characterized in that said predetermined signal gener¬ ator (340) is * adapted to generate a signal representing a repeated predetermined type binary digit. 6. A digital system according to claim 1, characterized in that said switching means includes a multiplexer device (320). 7. A digital system according to claim 3, characterized in that said data signal represents first and second type binary digits by respective lower and higher frequency signal level changes and wherein said clock signals have a frequency twice that of said higher frequency signal. OMPI

Inventors: DUKE JACK RONALD, DUKE, JACK, RONALD
Applicants: NCR CO, NCR CORPORATION
Application year: 1983
Main classification: G06F3
IPC codes: G06F3, G06F12, G06F13, H04L1