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

The functions of the five bytes in an active frame are described in the following paragraphs.StartThe start byte, which contains hexadecimal 39, signals the start of a frame cycle. The endof an active frame is indicated by the selected area station reaching a byte count of five.The start byte, generated by the 2790 Control, is the only byte that requires even parity.Area Station AddressCoded information in this byte can be used to select a unique area station, any area station,or all area stations.There are 128 unique area station addresses, which must be indicated by codes within therange of hexadecimal 80 to hexadecimal FF. All address codes outside this range are invalid.Because of the characteristics of the system, it is recommended that area station addressesbe assigned sequentially starting with hexadecimal 80. This does not, however, restrict thephysical placement of area stations on the loop.An any-area-station address is designated by a code of hexadecimal 11. Whenever an areastation requires service from the program, it searches for a frame having this code. Uponfinding one, the area station captures the frame by inserting its own address in place ofthe any-area-station address in this byte.An all-area-stations address is designated by a code of hexadecimal 09. Every areastation executes the instruction specified in the control byte, if it can, withoutmodifying the area station address byte.A software idle is designated by a code of hexadecimal 18. This code is used by the2790 Control programming to reserve a specific frame. No action is taken by the areastation.Device AddressA frame addressed to an area station is also addressed to a particular device (such asa 2795 Data Entry Unit) on a specific adapter connected to that area station. Bits 0 and 1of the device address byte identify the particular adapter in the area station, ordetermine that this byte is a part of diagnostic controls. Bits 2 to 7 identify the deviceattached to the addressed adapter. The diagnostic controls are normally addressed byhexadecimal 00. However, a diagnostic command is executed regardless of the contents ofthis byte. If an area station adapter is selected, or an area station captures an any-addressframe, the area station inserts its own address and the address of a particular deviceinto the frame.ControlThe control byte is divided into two hexadecimal digits. The low-order digit (bits 4 to 7)contains the command issued by the 2790 Control. The command digit is further subdividedinto an operation code (bits 6 and 7) and a modifier (bits 4 and 5).The high-order digit (bits 0 to 3) contains the response from an area station. An addressedarea station always responds to a command that is valid for that particular area station.Bit 0 of the response digit is the restore acknowledge bit and is nearly always set to 0.Bit 1 is the data mode bit. It is set on when the area station decodes one of the diagnosticcommands, or when the addressed adapter is in the data mode and decodes a valid operationcode. Bits 2 and 3 are status bits, and are defined by the area station to reflect thestatus of an operation in the area station adapter.The control byte codes permit two-way communication between the area stations and thesystem controller for every step during the various operations.II 111111111111m pm Iv,-