Systems Reference Library - All about the IBM 1130 Computing ...

is detected. Exit from transparent mode is accomplishedby placing a DLE followed by an ITB, ETB,ETX, or ENQ in the next-to-last word of a transmitdata table. When the line adapter receives the DLEfrom core storage, the byte counter is decreased to3. The combination of a DLE and a byte count of 3prevents insertion of the additional DLE. Therefore,a control sequence is detected.DLE SYN sequences are used to maintain synchronismin transparent mode in place of SYN SYNsequences used in normal mode. Frequency of insertiondepends on the line speed. DLE SYN sequencescannot be used by the program as time fills in transparentmode.Timeout ControlsTimeouts are used to ensure efficient utilization ofthe communications line and to prevent tie-ups due tofalse sequences or missed turnaround characters.Some timeout conditions cause an interrupt, butthe program has the option to suppress the timeoutinterrupt.TRANSMIT TIMEOUT (No Interrupt): This timeout isused to automatically insert the synchronous idlesequence in the output data stream as follows:1. In normal mode, and also in transparent modewith external clocking, a SYN SYN or DLE SYNsequence is inserted every 1.00 (±0.15) sec.2. In transparent mode with business machineclocking, a DLE SYN sequence is inserted atone of several possible intervals, depending onthe line speed:600 baud: 900 (±100) ms (WTC only)1200 baud: 475 (±50) ms2000 or2400 baud: 255 (±25) ms4800 baud: 106 (±12) msIn either case, insertion of the synchronous idlesequence is delayed if insertion would occur between:TIMEOUT (Interrupt): This timeout has the followingpurposes:• Limits the waiting time allowed for a transmittingstation to receive a reply (3 seconds).• Monitors incoming or outgoing data for SYN patterns.A timeout interrupt will occur in3 seconds if any of the following conditions occurin the data stream:1. A (SYN SYN) sequence is not decoded innormal mode.2. Continuous SYN characters are decoded innormal mode.3. A DLE SYN sequence is not decoded intransparent mode.4. Continuous DLE SYN sequences are decodedin transparent mode.• Causes a timeout interrupt if the data set fails torespond to a request-to-send with a clear-to-sendwithin 3 seconds.The timeout interrupt can be changed to a nominal2 seconds by issuing an XIO initialize write or XIOsense device, and specifying continue timer (modifierbit 10 on) in either case. If this is done, the timeout ischanged from 3 seconds to 2 seconds and the timer isstarted. The new timeout period remains effectiveuntil changed by one of the following events:1. A timeout interrupt occurs.2. A change from receive to transmit, or vice versa,occurs because of data table chaining or anotherXIO initialize write.3. An XIO initialize write or XIO sense device withmodifier bit 12 (clear CA) on is given.A timeout interrupt can be suppressed if bit 3 ofthe data table byte count word is on.Actual time periods for timeout interrupt are:Time Period Secondary Station Primary Station3-second timer3.0 seconds2.7 seconds• A DLE and its following control character.• An ITB character and the following BCC's.• The block check characters (BCC's).2-second continue timerall times ± 100 milliseconds2.0 seconds1.9 seconds111735 AIn the case of all non-ITB ending sequences,insertion is abandoned. Timeout is restarted whenthe synchronous idle sequence (SYN SYN or DLE SYN)is detected in the message stream.LINE ADAPTER INITIALIZATIONBefore a line adapter can transmit or receive information,it must be initialized to a state in which it isCommunications Adapter 185