Systems Reference Library - All about the IBM 1130 Computing ...

Page of GA26-5918Revised July 14, 19By TNL: GN26-02(via data channel operations as required. The 2790Iadapter uses the data channel for control purposes,as well as data transfer, by communicating with acontrol table in core storage.The control table contains one 16-word loopchannel control block (LCCB) for each of the 13 loopchannels, and one loop channel interrupt block(LCIB). The LCCB's contain control and status informationwhile the LCIB contains interrupt statusfor all 13 loop channels. Detailed descriptions ofthese control blocks are found under "Loop ChannelControl Block" and "Loop Channel Interrupt Block."To implement data transfer, the program initializesthe LCCB for the particular channel by specifyingthe type of operation (read or write), the corestorage address of the table containing the data, andthe number of bytes of data. The program activatesthe channel by turning on a control bit in the LCCB.The adapter then performs the data transfer operationon the active channel, using data channel operationsto communicate with the LCCB. The programis released for other processing.During a write operation, the adapter uses thedata buffer address and byte count in the LCCB tofetch data (one byte at a time) from the data bufferfor subsequent transmission to a device on thetransmission loop. Once initiated, a write operationcontinues until the byte count is decreased to 0, oran error occurs.During a read operation, the adapter uses thedata buffer address and byte count in the LCCB tostore incoming data in the data buffer (one byte ata time). Normally, a read operation is terminatedby a request from the input device. However, a readoperation is also terminated if the byte count isdecreased to 0, or an error occurs.LOOP CHANNELSEach adapter cycle, used in transferring informationaround the transmission loop, is divided into ninebasic time periods of equal duration. The adapterutilizes these nine time periods to implement a channelconcept for data transfer.High-Speed Loop ChannelsThe first eight time periods of an adapter cycle areused as loop channels 1 through 8 and are known ashigh-speed loop channels. Each of these eight loopchannels has an effective data rate of approximately100 characters per second. All eight high-speedchannels are sequentially selected for service duringeach adapter cycle.Low-Speed Loop ChannelsThe ninth time period of an adapter cycle is used toaccommodate five additional loop channels (9 through13). This is accomplished by sequentially selectingeach of the five loop channels (9 through 13) forservice during the ninth time period in five consecutiveadapter cycles. During the first cycle, forexample, channels 1 through 9 are serviced; duringthe next cycle, channels 1 through 8 and 10 areserviced. This sequence continues until channel 13is serviced, and is then repeated.Because each one of channels 9 through 13 isselected only once during each five adapter cycles,the effective data rate for these five channels isapproximately 20 characters per second. Therefore,channels 9 through 13 are known as low-speed loopchannels.Both high- and low-speed channels may be usedfor read and write operations. However the data rateof the input or output device must be considered duringan operation. For example, printer output to the 1053Printer must be transmitted via a low-speed channeland the card reader or DEU input must be transmittedvia a high-speed channel.Data Transmission FormatInformation is transmitted serially by bit around thetransmission loop at an approximate rate of 514 kHzas long as the adapter is in active mode. The informationbits are segregated into bytes containing eightbits plus parity. During each channel time frame, asequence of 30 bytes (called a channel frame) istransmitted to the loop. The first byte (byte 0) in achannel frame always contains a start character. Ifthe channel is inactive, the remaining 29 bytes containsync characters. If the channel is active, bytes1 through 4 are used to carry information, and theremaining bytes (5 through 29) contain sync characters.The channel frame format and its relationship tosequential channel transmission are shown in Figure120. Each byte except the start byte should containodd parity. The information bytes of the active channelframe are described in the following paragraphs.START BYTE: The start byte (byte 0) is the firstbyte transmitted in a channel frame and always containsa start character (/39 with even parity). Thestart character is automatically generated by theadapter at the beginning of each channel frame. Thestart byte is used to indicate start of frame and tomaintain byte synchronization.2790 Adapter 225