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

Data TransmissionInformation is transmitted over the loop by data bits, represented as bipolar pulses atapproximately 500 kHz, as long as the 2790 Control in the 5012 Multifunction Module haspower. All communication between the System/7 and the area stations is accomplished bya sequence of five information bytes, called a frame, shown in Figure 13-3. Byte 0 (thestart byte) is generated by the 2790 Control; the remaining four bytes (area station address,device address, control, and data) are generated by the program in the processor module.Figure 13-3 shows that 25 synchronization bytes separate each five-byte active portion in aframe cycle, for a total of 30 bytes. These 30 bytes make up what is called an active frame.All synchronization bytes are generated by the 2790 Control.Byte 0B O B 1 B2 B 3 B4 B 5 B6 B 7 B OByte 1 PByte 2 P Byte 3P Byte 4B7 B OB 7 B OB7 B OB 7 PStart AS address Device address Control Data(even parity) (odd parity) (odd parity) (odd parity) (odd parity)Figure 13-3. Data transmission formatFrames are transmitted by the 2790 Control at intervals of 524 microseconds. Activeframes can be separated by one or more inactive frames, which consist of a start byteand 29 synchronization bytes. If the controlling program does not specify a new activeframe, an inactive frame is sent out by the 2790 Control. This separation is usedprimarily to allow time to process input frames and to prepare output frames. The framefields are summarized in Figure 13-4.When multiple 2790 Controls (up to 4) are installed in a System/7 configuration, theoutput frames transmitted by each 2790 Control are offset by 3 bytes from the outputframes transmitted by any other 2790 Control. In other words, the first 2790 Controlstarts transmitting output frames. When byte 3 of its first output frame is sent, then thesecond 2790 Control starts transmitting. When byte 3 of its first output frame is sent,then any third 2790 Control starts transmitting. When byte 3 of its first output frameis sent, then any fourth 2790 Control starts transmitting. Once begun, the transmissionof output frames from all 2790 Controls is continuous.IBM 5012 Multifunction Module Model Al 13-27