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

output device and then returned to the adapter for avalidity check.The device error sub-sequence of a write sequenceis performed if the output device determinesthat it cannot perform the write operation, or if theoutput device detects an error condition during thewrite operation. In either case, sequence unusualend is posted in the channel sense word, a channelinterrupt to the program is generated, and the channelis deactivated. The data/status/guidance fieldcontains the unusual status received from the device.The end sub-sequence of a write sequence isperformed when the output device requests anotherdata character and the byte count is 0. The endsub-sequence posts the ending condition in the channelsense word, generates a channel interrupt to theprogram, and deactivates the channel. The data/status/guidance field contains the terminating statusof the device.Broadcast SequenceThe broadcast sequence is a modified write sequence.It is used to transfer data, such as time display update,from core storage to all AS displays. Thebroadcast sequence is entered by initializing anLCCB for a broadcast write operation. Duringbroadcast mode, no information in a channel frameis modified by any of the area stations. Therefore,no responses are returned by an AS. Each AS, ifable, performs the operation.Each channel frame in a broadcast sequence ischecked by the adapter to ensure that no informationhas been modified during transmission around theloop. If an error is detected, the condition is postedin the channel sense word, a channel interrupt to theprogram is generated, and the channel is deactivated.The broadcast sequence consists of three parts:(1) a write command to place all AS displays in datamode, (2) a series of write data commands to transferthe data characters to the AS display, and (3) awrite end command (after the byte count is decreasedto 0) to remove the AS displays from data mode andterminate the operation. When the operation isterminated with write end, the ending condition isposted in the channel sense word, a channel interruptto the program is generated, and the channel isdeactivated.Control SequencesControl sequences are used primarily for diagnosticpurposes. The adapter does not automaticallyprocess control command-response sequences.Therefore, unconditional interrupt must be used inconjunction with any of the control command sequences.Unconditional interrupt causes returningchannel frames to be stored in the LCCB errorframe field. Analysis of each returning channelframe must be performed by the program.INTERRUPT CONSIDERATIONSTwo types of interrupt conditions are associated withthe 2790 adapter: interrupts associated with loopchannel operations, and interrupts associated withoverall adapter operation.When an adapter interrupt occurs, the programshould issue an XIO sense device with modifier bits13 through 15 set to 000 to read the device statusword (DSW1) into the accumulator. If the adaptererror indicator (bit position 0) is on, it indicatesthat the interrupt is due to an error condition withoverall adapter operation. The error condition isindicated in bit positions 1 through 5 of DSW1. If thechannel interrupt indicator (bit position 7) is on, itindicates that the interrupt was caused by one ormore of the 13 loop channels.When a channel interrupt occurs, the interruptstatus word (ISW) located in the loop channel interruptblock (LOB) must be interrogated to determinewhich of the channels requires service. The channelsense word for each interrupting channel can then beinterrogated to determine the cause of the channelinterrupt request.After servicing all interrupt conditions indicatedin DSW1 and all channel interrupts as indicatedin the ISW, the program should issue an XIO sensedevice specifying DSW1 with reset (modifier bits 13through 15 set to 001). This resets the programresettable indicators in DSW1 and allows any channelinterrupt requests stacked in the interrupt statushold word (ISHW) to be transferred to the ISW. Threenonconsecutive machine cycles are used to transferthe contents of ISHW to ISW by means of data channeloperations. If any channel interrupt requests werestacked in the ISHW and transferred to the ISW, a newchannel interrupt to the program is generated, providedthe adapter is active.DIAGNOSTIC FUNCTIONSThe 2790 adapter provides several diagnostic functionsfor analyzing malfunctions that may occur.Diagnostic functions are enabled only when all segmentsof the transmission loop have been bypassed.2790 Adapter 251