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

DataThe data byte contains either data or status information, depending upon the contentsof the control byte.A data transfer caused by a read command always involves receiving the data byte twicefrom the area station, once in each of two frames. Thus, input data can be checked foraccuracy without special checking logic in the area station.Data transferred as a result of a write command is also checked for accuracy bytransmitting it to an area station and then receiving the data back from the area station.If the data received is the same as the data transmitted, the area station is thencommanded to print the data or turn on a specified indicator light.Commands and ResponsesThe control byte of each transmission frame contains coding and responses to all commands.All commands, which are generated by the System/7 program, are sent from the 2790Control to an area station, whereas all responses are returned from an area station to the 2790Control. Thus, a control byte transmitted from the 2790 Control should contain a responsewhen that command is received in the 2790 Control.Each command and response is represented by two hexadecimal digits corresponding to thecoding within the eight-bit control byte. The first hexadecimal digit (four high-order bitsof the control byte) is called the area station response digit. The second hexadecimal digit(four low-order bits of the control byte) is called the command digit. This command digitis further divided into a two-bit modifier and a two-bit op code as shown in Figure 13-5.Bit 0 of the response field is turned on to indicate that a frame is being sent by the 2792to the remote loop and a response is pending.Command frames are easily distinguished from response frames because the response digitis zero for all commands, but nonzero for all responses as shown in Figure 13-5. Thereare several commands in each of the three major command categories: read, write, andcontrol. In addition, there is at least one valid response for each of the commands.Note: When power is initially applied to the area stations, they are in the bypass modeof operation. (This mode is explained later in this chapter.) The controlling program mustissue control commands to restore discrete area stations, or a single control commandaddressed so as to restore all area stations. The restore operation inserts the areastation(s) into the transmission loop.1 3 1,111R! WUTIMIV! !PM linpqr