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

character (bit positions 0 through 7) is transferred tothe common bus, and the channel address register(CAR) is not advanced. When the byte counter containsan odd count, the low order character (bitpositions 8 through 15) is transferred to the commonbus, and the CAR is advanced.In receive mode, data flow is from the communicationline to the data set, to the SERDES, to thecommon bus or eight-bit buffer register, to corestorage. The 8-bit buffer is used to pack two 8-bitcharacters into one 16-bit core storage word. Whenthe byte counter contains an even count, the eight-bitbuffer is loaded from the SERDES and the channeladdress register (CAR) is not advanced. When thebyte counter contains an odd count, the character inthe SERDES is transferred via the common bus tocore storage , along with the character in the eightbitbuffer, and the CAR is advanced.Data transfer between core storage and SERDESis parallel by byte and serial by character. Datatransfer between SERDES and the communicationsline is serial by bit.CA Transmission CodeThe CA uses either EBCDIC (Figure 93) or ASCII(Figure 94) for transmission code. Both codesutilize eight bits per character. (ASCII uses sevenbits plus a parity bit.) Because a core storage wordconsists of 16 data bits, CA characters are packedtwo characters per word in core storage; one characterin bit positions 0 through 7 and one character inbit positions 8 through 15. All characters are transmittedlow order bit first as follows:Core Storage WordBit Positions0 1 2 3 4 5 68 9 10 11 12 13 14 15EBCDIC 0 1 2 3 4 5 6 7ASCII P 7 6 5 4 3First Bit to Line111730A1Communications control characters and sequencesdefined for the BSC system are listed in Figure 95 anddescribed in the following paragraphs.SOH (Start of Heading): SOH is used as the first characterin a block of heading data in a message. A headingmay consist of a series of noncontrol characters,which may be used to process the text portion of amessage. If a heading is subdivided into shorter transmissionblocks by use of ETB or ITB, SOH is used tostart any block that continues the transmission of theheading. Use of SOH following ITB is optional, butshould be used only if the block ended by ITB washeading.STX (Start of Text): STX is used to indicate the startof the text portion of a message and to terminate aheading. Text is that portion of a message treated asan entity to be transmitted to a remote station withoutchange. If text is subdivided into shorter transmissionblocks by use of ETB, STX is also used to starteach block of text that continues transmission of thetext. Use of STX at the start of a text block followingan ITB is optional.ITB (Intermediate Transmission Block): ITB is usedto block data by indicating end of intermediate text orheading without causing line turnaround. Block checkcharacters (BCC) are immediately transmitted followingan ITB. Another message block follows the ITBand BCC. SOH or STX may be included or omitted asthe first character of the block following an ITB, providedthe type of information (heading or text) in thetwo blocks is the same. However, SOH normally doesnot follow a previous text block, only a previous headingblock.ETB (End of Transmission Block): ETB is used toindicate the end of a transmission block which is notthe end of the message. ETB is followed immediatelyby the BCC's and calls for a response from thereceiving station. After an affirmative response,transmission of the message is resumed with an SOHor STX, depending on whether the ETB came withinthe heading, between the heading and text, or withinthe text.Control CharactersTen communications control characters provide thecontrol character set to implement control functions.ETX (End of Text): ETX is used to terminate a textor message. ETX is followed immediately by theBCC's and calls for a response from the receivingstation. After an affirmative response, transmissionmay continue with SOH or STX. ETX should not beused to terminate a heading.180