BYPASS MECHANISM FOR DAISY CHAIN CONNECTED UNITS

Abstract: Bypass mechanism for improving the operation of a daisy chain line linking a plurality of units and used for propagating a signal which may be trapped by the first unit in the chain having a pending service request to be serviced by a common resource 12. The daisy chain line 22 is connected to the units through input line IN-22 and the signal to be propagated, if not trapped by the unit is sent to the next unit through output line OUT-22. All units in the chain are connected to bypass line 26. Only one line is shown, but a plurality of lines may be provided when a plurality of sets of consecutive units should be able to be disconnected from the chain. When it is desired to disconnect consecutive units from the chain, gating logic 24 in the unit preceding the first unit to be disconnected causes the signal to be propagated, to be provided to bypass out line through 26-OUT and in the unit following the last unit to be disconnected, the signal to be taken from bypass in line 26-IN. This is done under the control of bypass control signals which are sent to the units through bus 14.

Description: This invention relates to a bypass mechanism preventing the continuity of a daisy chain to be interrupted when the units connected to this chain become inoperative. Background art The daisy chain connection of a plurality of units working under control of a central control unit is a conventional way of operation for selecting a unit in the chain. For example, a communication controller which processes the data to and from the communication lines, comprises a central control unit and several types of adapters such as the line adapters, the channel adapters, etc. A network control program NCP is running in the central control unit which controls the operation of the adapters through program initiated operation or adapters initiated operations. The adapters of each type, for example the channel adapters are connected through a common line in a daisy chain mode of operation. The adapters communicate with the central control unit through interrupts. The central control unit services the interrupts raised by the adapters. The selection of each adapter may be done either by the control program through a specific command or by an autoselection mechanism involving hardware. The autoselection mechanism is started on a broadcast command to the adapters. On this command, the adapters change the selected one, use the daisy chain line to perform the selection of any connected adapter having an interrupt pending. In such an environment, if an adapter is failing or has to be disconnected from the daisy chain, the daisy chain operation is interrupted. US patent 4 380 052 describes a mechanism which allows such a problem to be solved. According to this patent, each unit Pn comprises an alert circuit which is responsive to the arrival of the signal to be propagated in the chain, to the preceding unit P(n-1) to cause a signal to be restarted to the next unit P(n+1), if said unit is not to be selected and has not received the signal to be propagated within a fixed time duration. This mechanism presents the following drawbacks: It does not operate if more than one unit are failing and have to be bypassed, since if two or three units in series are failing the signal from the previous one is no more received. Furthermore, if a failing unit restarts its operation, there may be two signals which are propagated in the chain. GB-A-2088598 describes a system which permits only bypassing of one set of units. The bypassing function is controlled thanks to the propagation of forward and reverse propagation signals by each unit which presents the drawback of needing extra wires. Summary of the invention An object of the present invention is a simple and efficient mechanism implemented in a daisy chain arrangement of units which allows a desired maximum number of distinct non contiguous sets comprising a variable number of contiguous units to be bypassed when said unit(s) is (are) failing or to be disconnected from the chain. The bypass mechanism according to the subject invention improves the operation of a daisy chain line linking a plurality of units which can exchange command signals with a central control means (12) through a bus (14) said daisy chain line being used for propagating a selection signal which may be trapped by the first unit in the chain having a pending service request to be serviced by the central control means (12). The daisy chain line is connected to the units through input means (56) and the selection signal, if not trapped by the unit, is sent through output means (54) to the-next unit in the chain. According to the present invention, the bypass mechanism allows a desired maximum number x of distinct not contiguous sets identified through the central control means and comprising a variable number of contiguous units to be disconnected from the chain. The mechanism comprises the following means : x bidirectional lines (26-1, 26-2) each of which is attached to each unit, and which are terminated by their characteristic impedance, each unit having the capability to conditionally receive the selection signal to be propagated from said bidirectional lines and to conditionally provide the signal to be propagated to said bidirectional lines, the central control means (12) sending bypass out command signals through the bus to the units preceding the first unit in each identified set and bypass in command signals through the bus to the units following the last unit in each identified set, said bypass out command signals and bypass in command signals being indicative of selected lines among the x bidirectional lines to be used for bypassing each identified set of units, first bypass control means (50, 70-1, 70-2) located in each unit and which are responsive to the bypass out command signal received from the central control means to cause the selection signal to be sent to the selected bidirectional line as indicated by the received bypass out command signal, second bypass control means (60, 74-1, 74-2) located in each unit and responsive to the bypass in command signal received from the central control means to cause the selection signal to be received from the selected bidirectional line as indicated by the received bypass in command signal so as to bypass each set of units by means of a selected bidirectional line. Brief description of the figures Figure 1 shows the block diagram of the part of a communication controller wherein the mechanism of the present invention is embodied in the specific case where only one set of units (x=1) can be bypassed. Figure 2 shows in more details the gating control circuit 24 of figure 1. Figure 3 shows the gating control circuit to be used in the preferred embodiment of the present invention for the general case where x is higher than 1. Detailed description of the invention The detailed description of the invention will be made assuming that the units which are connected to the daisy chain line are the channel adapters of a communication controller. The channel adapters CA-1 to n are connected to the central control unit CCU 12 through bus 14. The central control unit 12, under control of programs in control store 16, controls the transfer of data from data store 18 to attached central processing units CPU (not shown) through channel adapters CA. The adapters comprise the conventional adapter logic circuitry 20 which contains buffer registers, address recognition circuits and sufficient control circuits working under the control of the central control unit to supervise the data transfers. This logic circuitry will not be described in details since it is not involved in the operation of the mechanism according to the present invention. As an example it may be similar to the circuitry which is implemented in the IBM 3705 or 3725 Communication Controllers. The adapters are connected through a daisy chain line 22. This gives a chance to each adapter to be serviced to avoid the loss of information. The adapters communicate with the central control unit through CCU interrupts which are processed by the CCU at a given level of the programs. The adapters raise an interrupt by activating the INT line of bus 14. When the CCU services the interrupts, a process of priority in the daisy chain allows one of these adapters to be selected until it completes its task. Then another selection can be made. The present invention does not relate to the priority processing, but to a bypass mechanism which is implemented in gating control logic 24 and makes use of bidirectional bypass line 26. This mechanism is used when one or several adapters are failing or to be disconnected from the chain. At the initialization time, one adapter is selected by the program running in the central control unit, for example adapter 2, to service an interrupt presented to CCU 12. The program issues a broadcast command on bus 14 which is decoded by every adapter. The selected one (adapter 2) upon decoding of this command activates the daisy chain out line at output 22-OUT of logic 24. The service request signal is also provided on line 28 to logic 24. The next adapter in the chain: adapter 3, has its input 22-IN active. If a service request is pending in this adapter, circuit 24 traps the IN signal on line 22 and does not activate the output 22-OUT. It completes the command and becomes the selected adapter. The control program issues several commands to the selected adapter to process its request. At the completion of the task, the control program resets the interrupt line in the selected adapter. On the next broadcast command, the same operations are performed. This is the normal way of operation in daisy chained adapters. If for any reason, power down, internal error, broken cable, etc... one adapter is no more able to complete its task or to propagate the signal on the daisy chain line, all adapters downstream in the chain are no more able to operate. The adapter comprises several checkers to verify the proper operation of the daisy chain. Any error is reported to the control program. The error processing routine determines the failing adapter by issuing a read sense command to the adapters. The adapters which have detected an error, answer the command with the sense data and their addresses. The failing adapters have to be removed from the daisy chain line. It will be assumed for the sake of explanation, that adapter 4 is the failing adapter in the chain. The program issues an output bypass command on bus 14 to adapter 3 preceding the failing adapter in the chain, which causes circuit 24 to activate the bypass OUT line 26. It also issues an input bypass command on bus 14 to adapter 5 following the failing adapter which causes the functional input line IN-22 of the daisy chain to be discarded and replaced by the bypass IN line 26 in circuit 24. By this way, adapter 4 is bypassed and does not affect the operation of the chain, even if its line 22-OUT is active. Adapter 5 monitors the bypass signal and drives its line 22-OUT if no service request is pending in this adapter. Control lines 30 from adapter logic circuitry 20 are activated to cause the bypass commands issued on bus 14 to be provided to circuit 24 of adapters which have to drive or monitor the bypass line. The bypass line 26 is a bidirectional line and is only driven by one and only one adapter, namely the one preceding the failing adapter and taken as input only in one adapter, namely the one following the failing adapter. The bypass line 26 is loaded at both ends by its characteristic impedance Zc, which facilitates the transmission of the bypass signal on the bypass line. In case adapter n, i.e. the last adapter in the chain is failing, adapter (n-1) drives its bypass out line and adapter 1 monitors the bypass in line instead of input line IN-22 of the functional daisy chain. A plurality of consecutive adapters may be bypassed by addressing the bypass out command to the adapter preceding the first failing adapter in the chain and the bypass in command to the adapter following the last failing adapter. If more than one disconnection of consecutive adapters is desired, a number of bypass lines equal to the desired number of disconnections is to be provided as shown in Figure 3. It will now be described in reference to figure 2, a logic arrangement which may be used to perform the bypass function in circuit 24. In figure 2, the same references as in figure 1 are used to designate the same components. Adapter logic circuitry 20 comprises a command decoding circuit 40 which is activated by a signal on control line 31 when the central control unit sends a -command through bus 14 to the selected adapter. Circuit 40 decodes the commands and generates therefrom an active signal on BYPASS IN COMMAND line 46 or BYPASS OUT COMMAND line 48 as the case may be or on the initialization line 42 to start the selection mechanism. Signal on initialization line 42 is active when the central control unit issues a broadcast command on bus 14 to start a selection operation. Line 42 is provided together with the ADAPT SELECT signal on line 44 to the trap and control logic 45. In the selected adapter logic 45 activates its output line 47. In normal mode of operation, (no failing adapter), BYPASS OUT COMMAND line 48 is inactive. The signal on line 48 is inverted in inverter 50, the output signal of which conditions AND gate 52 and driver 54 to activate the daisy chain line through output 22-OUT. The next adapter in the daisy chain gets its line IN-22 active. The signal is applied to AND gate 58 through receiver 56. The BYPASS IN COMMAND line 44 is not active, thus inverter 60 provides an active signal which conditions AND gate 58. Thus AND gate 58 transmits to OR gate 62, the incoming signal on line 22 to trap and control logic 45. If the service request line 28 is inactive, trap and control logic 45 activates line 47 so that AND gate 52 which is conditioned since the BYPASS COMMAND OUT line 48 in this adapter is inactive, propagates the signal through line 22-OUT to the next adapter. If the service request line 28 is active, trap and control logic 45 does not activate output line 47, but activates the NEW SELECT line 68 to the adapter logic circuitry 20. Adapter 20 completes the sequence on bus 14. The BYPASS OUT mode is set in the adapter preceeding the failing ones in the daisy chain. Upon detection of a BYPASS OUT COMMAND in circuit 40, line 48 is activated. Thus AND gate 52 is not conditioned and AND gate 70 is conditioned. The signal on line 47 coming out trap and control logic 45 is propagated through driver 72 on bypass OUT line 26-OUT The BYPASS IN mode is set in the first adapter which follows the last failing adapter in the daisy chain. In this adapter the command decode circuit 40 upon detection of a BYPASS IN command, activates line 46. Inverter 60 provides an inactive signal which prevents the IN signal received from 22 by receiver 56 from being propagated to trap and control logic 45 by AND gate 58. AND gate 74 has one input connected to the output of receiver 76 which monitors the bypass IN line 26-IN and is conditioned by the active signal on line 46. The output of AND gate 74 is provided to OR gate 62, the output of which is connected to trap and control logic circuit 45. The logic shown in figure 2 allows one or several adapters in series to be bypassed, since the bidirectional bypass line may only be driven by one adapter (the one preceding the first failing adapter) and may only be monitored by one adapter (the one following the last failing adapter). In order to have two or more series of adapters bypassed, a corresponding number of bypass lines have to be provided. Figure 3 shows how the circuit of figure 2 has to be modified when two bypass lines 26-1 and 26-2 are provided. The same references as in figure 2 are used, a suffixes 1 or 2 being added to the components which have to be duplicated to drive and monitor the bypass lines 26-1 and 26-2. These components are gates 70 and 74 and bypass command lines 46 and 48. Inverters 50 and 60 are replaced by an inhibit circuit which prevents gates 52 and 58 from being conditioned, when bypass command lines 46-1, 46-2 and 48-1, 48-2 are active.

Claims: A bypass mechanism for improving the operation of a daisy chain line linking a plurality of units which can exchange command signals with a central control means (12) through a bus (14) said daisy chain line being used for propagating a selection signal which may be trapped by the first unit in the chain having a pending service request to be serviced by the central control means (12), said daisy chain line being connected to the units through input means (56) and the selection signal, if not trapped by the unit, being sent through output means (54) to the next unit in the chain, said mechanism being characterized by allowing a desired maximum number x of distinct not contiguous sets identified through the central control means and comprising a variable number of contiguous units to be disconnected from the chain, said mechanism further comprising : x bidirectional lines (26-1, 26-2) each of which is attached to each unit, and which are terminated by their characteristic impedance, each unit having the capability to conditionally receive the selection signal to be propagated from said bidirectional lines and to conditionally provide the signal to be propagated to said bidirectional lines, the central control means (12) sending bypass out command signals through the bus to the units preceding the first unit in each identified set and bypass in command signals through the bus to the units following the last unit in each identified set, said bypass out command signals and bypass in command signals being indicative of selected lines among the x bidirectional lines to be used for bypassing each identified set of units, first bypass control means (50, 70-1, 70-2) located in each unit and which are responsive to the bypass out command signal received from the central control means to cause the selection signal to be sent to the selected bidirectional line as indicated by the received bypass out command signal, second bypass control means (60, 74-1, 74-2) located in each unit and responsive to the bypass in command signal received from the central control means to cause the selection signal to be received from the selected bidirectional line as indicated by the received bypass in command signal so as to bypass each set of units by means of a selected bidirectional line. A bypass mechanism according to claim 1, characterized in that the first bypass control means comprise : inhibiting means (50, 52) which are responsive to the bypass out command signals to prevent the selection signal from being transmitted on the chain by the output means (54) when at least one of the said bypass out command signals is active, x gating means (70-1, 70-2), each one being responsive to a bypass out command signal to cause the signal to be propagated to be provided to the selected bypass line, as indicated by the bypass out command signal when said bypass out command signal is active. A mechanism according to claim 1 or 2, characterized in that the second bypass control means comprises : inhibiting means (60) which are responsive to the bypass in command signals to prevent the selection signal received by the input means (56) from being trapped or propagated by the unit when at least one of said bypass in control signals is active, x gating means (74-1, 74-2), each one being responsive to a bypass in command signal to cause the signal from a selected bypass line as indicated by the bypass in command signal to be trapped or propagated by the unit when said bypass in command signal is active.

Inventors: BILLIAN, AUGUSTE FRANCOIS JOSEPH, Billian, Auguste François Joseph
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G06F13
IPC codes: G06F13