IBM System/7 Functional Characteristics - All about the IBM 1130 ...

An area station requiring service looks for the any-address code and captures the frameby replacing the code with its discrete area station address. This not only identifiesthe area station requesting service, but also prevents any other area station from capturingthis frame. The area station then inserts the address of the device that will be servicedinto the device address field and inserts a read request response code in the controlbyte. The program then generates an outgoing frame containing the area station address,device address, a read command, and a 0 data byte. This transmission places the areastation in data mode, which permits the requesting device to transmit data. The area stationresponds to the read command, and the program sends out a read null command. The areastation responds to the read null command with a read data request when it has a databyte ready to be sent to the 2790 Control. This data byte accompanies the read data requestresponse. The program then sends out a read data command, to which the areastation responds with a read data acknowledge and, again, the data byte. This doubletransmission of the data byte allows for validation of every data byte transmitted by thearea station.Under normal operation the area station can initiate the read-end sequence when it hascompleted a data or transaction code entry. The sequence is started when the area stationresponds to a read null command addressed to it with a read end request and normal statusin the data byte. The controlling program then sends out a read end command with eithera guidance character or status information in the data field, depending on whether theaddressed device is a data entry unit or a device on the area station local I/O adapter.Guidance characters apply only to the local I/O adapter device.I/O CommandsThe prepare I/O command is used as described in Chapter 4 under "Input/OutputInstruction." Data transfers between the 2790 Control and the processor module areaccomplished by immediate read and immediate write commands. Modifier field bits inthe immediate read and write commands further define the operation to be performed.All I/O commands addressed to the 2790 Control function must have a device address of100000. The module address varies, depending upon the physical location of thedigital I/O module in the 5026 Enclosure. The 2790 Control has two 4-byte bufferregisters, one for input frames and one for output frames. Two immediate writecommands are used to fill the output buffer register with a frame for transmission,while two immediate read commands are used to read the frame in the input bufferregister. The start byte (the first byte of a frame) is generated by the 2790 Control, sothis byte is not loaded into the output buffer or read from the input buffer.Write Address0 5 8 11 16 20 26 31Op code R Fun Zeros Mod DA MA0 0 0 0 1 XXX 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 XXXXXX1111 ii 11 1i 11 III 1 1 1 1 1 11111s.......se......"*".•••■„,•••••■••■•■••.,,■•••■■■•••■„,.....,•■••••y■•••■•••••,■•••••••■■■■••.„,.....0,•■■,...••■••0 8-F 2 0 0 8 0-3 XThis command stores the area station address and device address bytes into the first twobytes of the output buffer register in the 2790 Control. These two address bytes aretransferred from the index register (R), or the accumulator if R=000.The write-address command must be issued once to establish the desired addresses inthe output buffer register. Any number of write-control-and-data commands, describednext, can then be issued without having to issue another write address command. Anotherwrite address command must be issued only when the address(es) must be changed.