TransCore encompass 4 - Introduction

Communications Protocols
6-3
Chapter 6
Communications Protocols
This chapter describes the communications protocols for the
Encompass
®
4 Reader.
Introduction
The Encompass 4 Reader supports the following communications protocols:
• Basic
• Error correcting protocol (ECP)
• Data inquiry
The following protocol information provides reference information relevant to devel-
oping host software.
Communications are performed using the 7-bit ASCII code with optional parity, thus
providing easy setup, testing, and diagnostics with standard ASCII terminals and
serial printers. Parity must be enabled to achieve the specified undetected error rate.
Each message is framed within the start-of-message (som) and end-of-message (eom)
characters so that the host computer can detect the beginning and end of each mes
-
sage. This convention is most important under marginal communications conditions
during which the host may receive extraneous noise-induced characters between
reader transmissions. In such instances, the host is capable of ignoring any messages
that do not conform to the som...eom frame sequence.
Both data mode and command mode require a two-way message interchange is when
using ECP. This interchange is completed by the message recipient returning a mes
-
sage acknowledgement to the message sender.
With ECP, all transmissions require a message. If a message is not received, the sender
will time out with the same effect as if it had received a negative acknowledgment
(from the host) or an Error message from the Encompass 4 Reader.
Software (XON/XOFF) flow control is optionally supported. Be careful in the use of
XON/XOFF since noise-induced characters may be interpreted by the Encompass 4
Reader as the XOFF character, which would suspend reader output without informa
-
tion reaching the host computer. For more information see “Software Flow Control”
on page 8-19.
Note: TransCore recommends that XON/XOFF flow control be disabled while using
ECP.