TransCore encompass 4, Руководство пользователя

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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 developing 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 message. 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 when using ECP. This
interchange is completed by the message recipient returning a message acknowledgment 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 information reaching the host computer. For more information see
"Software Flow Control" on page 8–145 .
Note:
TransCore recommends that XON/XOFF flow control be disabled while using ECP.
Chapter 6 Communications Protocols
TransCore Proprietary
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