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

Standard replies as well as NAK cause an endcharacter-decodedinterrupt allowing the programto initiate line turnaround.TURNAROUND SEQUENCES: These sequences areused to cause an end-character-decoded interrupt,thus allowing the program to initiate line turnaround.Turnaround sequences are formulated by a DLEas the leader, followed immediately by any of theturnaround characters indicated in Figures 93 and94The character sequence DLE turnaround can beused as an acknowledgment to which a specialmeaning may be assigned by prior agreement.Turnaround character sequences are not recognizedif SOH or STX has been previously detected inthe message stream.Data Transmission CheckingData checking is based upon transmission of additionalinformation with each transmission block ofdata (heading and/or text blocks). This extra informationpermits error detection by the receiving station.This information is called a block checkcharacter (BCC). The three checking methods usedto generate BCC's are described next.CYCLIC REDUNDANCY CHECK (CRC-16): Thischecking is performed automatically by the lineadapter and is used only with EBCDIC in both normaland transparent modes. CRC produces a 16-bitcheck character by an arithmetic accumulation ofthe message bits for each transmission block. This16-bit CRC character is transmitted as two contiguousBCC's immediately following the end-of-blockcharacter. The receiving station also produces itsown CRC character using the same method. This16-bit CRC character is compared for agreementwith the two BCC's received. If they do not agree,a BCC error is set in the device status word.CRC accumulation begins following the firstappearance of an SOH or STX in normal mode or anSOH or DLE STX in transparent mode. The SOH,STX, or DLE STX is not included in the accumulation.CRC accumulation ends with and includes theend-of-block character (ITB, ETB, or ETX). IfITB is the end-of-block character, CRC accumulationfor the next block begins automatically withoutan SOH or STX. All characters transmitted orreceived during accumulation are included, exceptthose characters that are not sent to core storage atthe receive station. These characters are: SYNSYN, DLE SYN, or first DLE in DLE DLE sequence.VERTICAL REDUNDANCY CHECK (VRC): VRC isperformed automatically by the line adapter and isused only with ASCII in normal mode. This codestructure defines an eighth bit to provide odd parityfor each character. A vertical redundancy check(parity) is performed on each character; if its parityis not odd, data parity error is set in the devicestatus word. The VRC check is made on all transmissions:responses, control sequences, headings,and text.LONGITUDINAL REDUNDANCY CHECK (LRC):LRC is performed automatically by the line adapterand is used only with ASCII in normal mode. TheLRC character is an eight-bit character. It consistsof the bits necessary to make the total numberof 1-bits even in each individual bit level of the codefor the entire transmission block. The LRC characteris transmitted as one BCC immediately followingthe end-of-block character. The receiving stationproduces its own LRC character, which is comparedfor agreement with the BCC received. Ifthey do not agree, a BCC error is set in the devicestatus word.LRC accumulation begins after the first appearanceof an SOH or STX. The first SOH or STX is notincluded in the accumulation. LRC accumulationincludes and ends with the end-of-block character(ITB, ETB, or ETX). If ITB is the end-of-blockcharacter, LRC accumulation for the next block beginsautomatically without an SOH or STX. All characterstransmitted or received during accumulationare included except those characters that are notsent to core storage (SYN SYN).Transparent Mode ControlTransparent mode allows the full 256 EBCDIC charactercodes to be transmitted as text. Transparentmode is used when binary information, or externalcodes are to be transmitted or received. No provisionfor transparent mode operation using ASCII isprovided.A line adapter enters transparent mode after aDLE STX sequence. In transparency, a transmittingline adapter inserts a second DLE after eachdata DLE received from core storage. For example,a DLE ETB sequence transmitted during transparentmode appears as a DLE DLE ETB sequence, and thecontrol character sequence of DLE ETB is not recognized.At the receiver, the DLE preceding theinserted DLE is deleted and not sent to core storage.A line adapter leaves transparent mode when asingle DLE followed by an ITB, ETB, ETX, or ENQ184