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Servicing Attached Devices on a Bus

IP.com Disclosure Number: IPCOM000089423D
Original Publication Date: 1977-Oct-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Bantz, DF: AUTHOR [+2]

Abstract

A conventional way for a channel to interface with control units is the use of the arrangement shown in Fig. 1. IBM System/360 and IBM System/ 370 use this arrangement [*]. The diagram shows that any device may request service by placing a signal on the Request line. The channel responds by sending a signal on Select-Out. Each device retransmits it or not depending on whether the device is requesting service. Finally, the Select-Out line becomes the Select-In line. The channel uses Select-In to check that the Select-Out was accepted by one of the devices. The arrangement described in the figure can be generalized to a set of attachments which require the service of a resource.

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Servicing Attached Devices on a Bus

A conventional way for a channel to interface with control units is the use of the arrangement shown in Fig. 1. IBM System/360 and IBM System/ 370 use this arrangement [*]. The diagram shows that any device may request service by placing a signal on the Request line. The channel responds by sending a signal on Select-Out. Each device retransmits it or not depending on whether the device is requesting service. Finally, the Select-Out line becomes the Select-In line. The channel uses Select-In to check that the Select-Out was accepted by one of the devices. The arrangement described in the figure can be generalized to a set of attachments which require the service of a resource. A main characteristic of such an arrangement is that Select-Out is delayed at each device, and therefore a large delay is experienced before Select-In is available for checking.

A different scheme is described herein which reduces the delay. This accelerated scheme seeks to partition the set of attachments into groups, and then quickly determines whether a group will contain a selected attachment. Thus, the propagation of Select-Out can be accelerated by bypassing unproductive groups. The carry-bypass adder provides an analogous example in bypassing carry signals around sets of digits whose values would propagate the carry.

Fig. 2 shows the device arranged in groups of four. A device requests service as in the conventional manner. When Select-Out is sent out, special circuits determine whether any of the group is requesting service. If none is requesting, then Select-Out is immediately transmitted to the next group. The diagram also shows that each device may send a Select-Acknowledge, when it receives Select-In and it has requested service. In this way the channel is notified immediately that a selecting device has received Select-In.

A more detailed look at the circuit for bypassing a group shows that the signal transmitted to the next group is the OR of two other signals. One of these signals indicates that the group is to be bypassed. The other signal is the conventional delayed signal which is transmitted from device to device. The OR of these signals is transmitted to protect against the case where the circuit detecting the bypass fails to produce the required signal. In that case the delayed Select-In will be transmitted through the OR block, and the remaining devices will receive the proper signal.

To analyze the efficiency of the accelerated scheme as compared to t...