Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 257)

AN93
Rev. 1.3 257
A PPENDIX A—EPOS A PPLICATIONS
EPOS applications generally require nearly flawless call connection reliability and a very short overall transaction
time. The message length of a typical EPOS terminal is between 120 to 260 bytes of information. Due to the
relatively small message length, the need for reliable connections under all line conditions, and short connection
times, the preferred modulations have traditionally been variations of V.22 (1200 bps) or Bell 212 (1200 bps).
EPOS servers do not strictly follow ITU standards. Despite the informal use of the term “V.22 fast connect”, there is
no ITU “fast connect” standard. De-facto standards with modifications of ITU standards, such as V.22 Fast
Connect, have been adopted to reduce the transaction time. Some server manufacturers make changes to the
modem with the intent of making it difficult for competing terminals to connect. Many EPOS servers have out-of-
specification clocks and use reduced handshake timing. V.22bis (2400 bps) is occasionally used in EPOS
terminals as well. The primary method by which V.22bis terminals achieve a shorter connection time has been
through the use of a shorter answer tone. V.29 FastPOS is a Hypercom proprietary protocol based on the V.29 Fax
standard. For these reasons, EPOS applications often require some amount of reverse engineering and fine tuning
of the ISOmodem performance, typically with a patch.
Recently, improvements to the overall user experience have necessitated the storage of transaction information
within the EPOS terminal based on some predefined criteria. These stored transactions are typically sent as part of
a larger transaction at a later time. This effectively increases the message length to over 2 kB, necessitating the
use of higher-speed modulations, such as V.29 FastPOS or V.32bis.
The choice of either V.29 FastPOS versus V.32bis is a tradeoff between transaction message length and
connection times. It is common for a terminal to support both of these modulations.
Modulation Typical Connect Time (Sec)*
V.90 25
V.34 10.7
V.32b 7.8
V.22b 5.0
V.22 3.0
V.22 FastConnect 0.6
V.29 FastPOS 0.5
*Note: Does not include dial delay.
VISA II (7E1)
Terminal
ACK (0x06)
Host
STX <data>ETX, LRC
Dials host
Disconnects
EOT (0x04)
STX <data>ETX, LRC
ACK (0x06)
ENQ (0x05)