Chapter 5 Communications Protocols
The AI1422E Reader supports the following communications protocols:
•
Basic
•
Error Correcting
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 AI1422E 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 AI1422E Reader as the XOFF character, which would
suspend reader output without information reaching the host computer. For more information refer to
“525 Display Communications Protocol Status” on page 6–36
.
Note: TransCore recommends that XON/XOFF flow control be disabled while using ECP.
Basic Protocol
With basic protocol, messages sent to and from the AI1422E Reader and the host are transmitted without
error checking. For each host transmission, the AI1422E Reader returns a
!Done
or
!Error
message to
the host.
When the host computer is physically close to the AI1422E Reader and no sources of interference exist,
the basic protocol provides reliable communications.
The host must be ready to receive reader-transmitted messages because in basic protocol the AI1422E
Reader does not wait for the host to acknowledge a message before transmitting the next message. If
necessary, the host may halt reader transmissions by using software or hardware. Refer to
Error Correcting Protocol
When the quality of data communications is imperative or may be suspect, you can invoke ECP to ensure
the integrity of data transmitted between the AI1422E Reader and the host.
Chapter 5 Communications Protocols
TransCore Proprietary
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