User`s guide

200 DC 900-1340I

BSC Programmer’s Guide

A.4.3 3270 Command Checking

A tributary link scans the incoming data stream for the presence of valid 3270 com-

mands for devices configured either as displays or printers. First, the link checks for the

presence of the escape (ESC) character that precedes each BSC 3270 command. Second,

it checks the 3270 command byte for a valid Read, Write, Copy, or Erase command. If

a bad command is found, line response and status maintained are determined by the

type of command error that occurred as shown in Figure A–6.

Normally, the BSC 3270 master sends EOT when it receives a WACK or RVI response

to a select sequence. If the BSC 3270 master sends a Write or Copy command instead of

EOT, the ICP discards the command, sends EOT to the BSC 3270 master, and sets the

appropriate status bits. Figure A–7 and Figure A–8 show two examples.

Figure A–5: Normal Operation of Virtual Display

Client Freeway

Communication Line

3270 WRITE command byte

select

with start print bit

<EOT>

<ACK0>

<ACK1>

DLI_PROT_SEND_NORM_DATA_EOM

dlRead