Simple Modification of a Non-Peer-To-Peer System Bus for Adapter-To-Adapter Communication
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
The system bus (UC bus) of the IBM 3725 Communication Controller allows only communications between a CCU (Central Control Unit) and adapters. This article describes a simple modification of this bus in order to allow adapter-to-adapter communication. In order to minimize modifications, the new mechanism used for adapter-to-adapter exchange takes advantage of the adapter-to-CCU exchange on the UC bus, known as Cycle Steal or AIO (Adapter Initiated Operation). The source adapter initiates an AIO operation as if it were working regularly with the CCU. When the CCU is ready to answer at this request with the Cycle Steal Grant (CSG) signal, the CCU places the data part of the UC bus in CCU receive mode (receivers are put on the CCU side). This allows the source adapter to use the bus for communication with the target adapter.