User`s guide

38 DC 900-1340I
BSC Programmer’s Guide
2.3 BSC 2780/3780 Protocol Implementation
The protocol used in the BSC 2780/3780 product is a logical, half-duplex, bisynchro-
nous, point-to-point protocol used by IBM 2780 and IBM 3780 remote job entry termi-
nals (sometimes called RJE terminals). Details on the operation of IBM’s BSC protocol
can be found in document GA27-3004, General Information — Binary Synchronous
Communications, IBM. Although this manual might imply that BSC 2780 and
BSC 3780 are different protocols, the line-control procedures of BSC 2780 and
BSC 3780 are exactly the same. The only difference is in the format of the text blocks.
Protogate’s BSC 2780/3780 product can send and receive both 2780 and 3780 text
blocks in normal and transparent modes. If you are not sure which protocol you’ll be
using, the following sections explain the differences between BSC 2780 and BSC 3780.
2.3.1 BSC 2780 Protocol
Figure 2–1 shows the normal 2780 text block. It begins with a start-of-text character
(STX character) and ends with an end-of-transmission-block (ETB) or end-of-text
(ETX) character. Each data record within the text block ends with a unit separator char-
acter (called ‘US’ in ASCII and ‘IUS’ in EBCDIC), except the last record of the block.
Each record is followed by a block-check character (called BCC) that is a redundancy
check (CRC-16 or LRC-8) of the characters in that record, including the US or
ETB/ETX character. The number of synchronization (SYN) characters are described in
Section 6.4 on page 161. PAD characters ensure complete transmission of the data
block.
Figure 2–1: Normal 2780 Text Block
SYN SYN STX record BCCUS US BCC record ETXrecord BCC PAD