User`s guide

2: BSC Protocol Summary
DC 900-1340I 39
Figure 2–2 shows the transparent 2780 text block. Each block starts with a data-link-
escape STX character pair (a DLE STX character combination) and ends with a DLE
ETB or DLE ETX character pair. Each record within the text block also starts with DLE
STX and ends with a DLE US. In transparent operation, two SYN characters must be
inserted after each block check character that follows DLE US.
BSC 2780 text blocks are normally a maximum of 400 bytes, which is the internal buffer
size of the IBM 2780 terminal. More details of the BSC 2780 protocol can be found in
the IBM document, Component Description: IBM 2780 Data Transmission Terminal.
2.3.2 BSC 3780 Protocol
Figure 2–3 shows the normal 3780 text block. It begins with a start-of-text (STX) char-
acter and ends with an end-of-transmission-block (ETB) or end-of-text (ETX) charac-
ter. Each data record within the text block ends with a record separator character (called
‘RS’ in ASCII and ‘IRS’ in EBCDIC), except the last record of the block where insertion
of an RS character is optional. Each text block is followed by a block check character (a
BCC) that is a redundancy check (CRC-16 or LRC-8) of the characters in the entire
block, starting with the first character following STX and ending with the ETB or ETX
character. The number of SYN characters are described in Section 6.4 on page 161. PAD
characters ensure complete transmission of the data block.
Figure 2–4 shows the transparent 3780 text block. Each block starts with DLE STX and
ends with DLE ETB or DLE ETX.
Figure 2–2: Transparent 2780 Text Block
SYN SYN DLE STX record DLE US BCC SYN SYN DLE STX record
DLE US … SYN DLE STX record DLE ETX BCCBCC SYN PAD