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

2791/2793 Adapter Critical timingInput adapter1035 Badge Reader Minimum of 2 frames* between aread data command frame and a readnull frame.Card readerData entry unitadapter0 Minimum of 2 frames* between aread data request frame and read datacommand frame.0 Maximum of 14 frames* between twoconsecutive read null frames (to avoidcharacter overrun condition in 2790).Maximum of 16 frames* between twoconsecutive read null frames (to avoidcharacter overrun condition in 2790).1053 Adapter 0 Normal rate of 42 frames per secondat equal intervals of 23.5 milliseconds.(Approximately 45 frames* between framesaddressed to this adapter).0 Minimum of 133 frames* between twoconsecutive write null command frames thatfollow a write data command frame.* Frames included in this count cannot address the adapteror device to which this timing applies.Figure 14-6. 2790 critical timings2792 Error StatusStatusCharacters(hex) Meaning00 Good status01 Open loop02 Data check04 Control check08 Parity error10 Communications data error20 Communications channel timeout40 All area station command error2790 Control OperationThe 2790 Control uses two 4-byte buffers—an output buffer and an input buffer. Theoutput buffer must be loaded with the area station address, device address, control, anddata bytes to be transmitted on the 2790 loop. When the loop is not actively transmittingdata (the four output buffer registers have not been filled by the program), a start bytefollowed by 29 sync bytes will be transmitted on the loop every 524 microseconds. When30 bytes have been transmitted and the output buffer register has been filled, the nextoutgoing frame contains the start byte followed by the contents of the four-byte outputbuffer register. Twenty-five sync bytes follow this active frame, and then another startbyte.Frame cycles transmitted from the output buffer are received back in the 2790 Controlin the four-byte input buffer. When the area station address, device address, control, anddata bytes are received in the input buffer, the 2790 Control presents an interruptionrequest to the System/7 processor. Errors are detected in the 2790 Control at the inputbuffer register.IBM 5013 Digital I/O Module Model Al 14-25