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

Write Null Acknowledge: This code (hexadecimal 41) is a response to a write null commandif no action is presently required by the area station adapter and the adapter is in thedata mode.Write Command Acknowledge: This code (hexadecimal 45) is a response to a writecommand. The response indicates that the area station adapter has received the commandand that the adapter, has inserted its status in the data byte. An all-zero status indicates thatthe operation will be executed. A nonzero status indicates busy, which can be returnedonly by the area station local I/0 adapter.Control CommandsAll control commands are generated by the program in the System/7 processor and sent tothe area stations by the 2790 Control.Bypass: This code (hexadecimal OB) can be used by the program as part of its diagnosticprocedures. The command causes the area station to remove its shift register from the loopafter the control byte has been received. This bypass operation eliminates the data bytefrom this frame, since there is a one-byte delay in the shift register. Since the area stationremoves the data byte from the frame, a short count results in the 2790 Control bytecounter. However, the start byte of the next frame resets the byte counter.While an area station is in a bypassed state, it can monitor the line, but it cannotinsert information. The only valid response is bypass acknowledge.Restore: This code (hexadecimal 03) is used by the program as part of its diagnosticprocedures. The command causes the bypassed area station to insert its shift registerin series with the loop after the control byte has been received. This insertion resultsin a repeat of the control byte (the restore command), which adds one byte to the activeportion of the frame. A skip character (all O's) is also substituted for the old data byte.The active frame returning to the 2790 Control contains six bytes instead of five. Thefourth and fifth bytes contain the restore command and the sixth byte contains theskip character. The skip character causes the 2790 Control byte counter to skip a count.If a restore command is sent to an area station that is not bypassed, the area stationwill return a restore acknowledge response, which enables the program to determine whetherthe command has been executed. The only valid response is the restore command itselfimmediately followed by a repetition of the restore command.Send Sync: This code (hexadecimal 04) may be used by the program after an area stationhas been placed in diagnostic mode. When an area station in diagnostic mode receives asend sync command, the area station sends a continuous stream of sync characters (eachcontaining hexadecimal 47) until it receives from the 2790 Control at least two charactersin succession containing hexadecimal FF. This can be achieved by sending a frame withhexadecimal FF in all byte positions. The only response to this command is the fact thatthe area station generates a stream of sync characters.The send sync command provides the ability to isolate a link (connecting two successivearea stations) that is failing intermittently and causing the system's maximum error ratecount to be exceeded.The command allows predetermined data (sync characters) to be constantly transmittedby a selected area station without that area station transmitting what it is receiving.2792 Reset. This code (hexadecimal OF) is used to reset circuitry in the 2792 for initialstartup and error reset conditions. It is recognizable only when used with an "all areastation" address (hexadecimal 09) and is ignored by 2791/93 area stations.IBM 5013 Digital I/O Module Model Al 14-21