3.3.2 Data Entry Terminals

DISCUSSION:

In addition to the line-oriented and character-oriented ASCII terminals for which Telnet was designed, there are several families of video display terminals that are sometimes known as "data entry terminals" or DETs. The IBM 3270 family is a well-known example.

Two Internet protocols have been designed to support generic DETs: SUPDUP [TELNET:16, TELNET:17], and the DET option [TELNET:18, TELNET:19]. The DET option drives a data entry terminal over a Telnet connection using (sub-) negotiation. SUPDUP is a completely separate terminal protocol, which can be entered from Telnet by negotiation. Although both SUPDUP and the DET option have been used successfully in particular environments, neither has gained general acceptance or wide implementation.

A different approach to DET interaction has been developed for supporting the IBM 3270 family through Telnet, although the same approach would be applicable to any DET. The idea is to enter a "native DET" mode, in which the native DET input/output stream is sent as binary data. The Telnet EOR command is used to delimit logical records (e.g., "screens") within this binary stream.

IMPLEMENTATION:

The rules for entering and leaving native DET mode are as follows:

• The Server uses the Terminal-Type option [TELNET:10] to learn that the client is a DET.

• It is conventional, but not required, that both ends negotiate the EOR option [TELNET:9].

• Both ends negotiate the Binary option [TELNET:3] to enter native DET mode.

• When either end negotiates out of binary mode, the other end does too, and the mode then reverts to normal NVT.