Welch Allyn SCANTEAM 3000PDF, Техническое руководство, Страница: 42 / 92

Programming the Reader
4–10
For example: suppose the receiver stores transmitted
characters in a 512Ćbyte buffer, yet the PDF417 reader has a
1400Ćbyte message to transmit. The receiver might take 510
characters into its buffer, then tell the reader to STOP
transmitting. As the receiver processes the message, space
becomes available in the 512Ćbyte buffer and the receiver can tell
the reader to START transmitting again.
Two methods of flow control have been implemented in the PDF
capableproducts: RTS/CTS hardwarehandshaking, and an
XON/XOFF protocol.
RTS/CTS Handshaking
This option uses two signals in addition to the transmit and
receive signals. The reader will have control of the RTS signal,
the receiver controls the CTS signal.
Whenever the reader has a message to send, it will raise the
Request To Send (RTS) line. If the reader has no messages to
send, the line will be held low. When the receiver wants to
suspend transmission, the Clear to Send (CTS) line will be held
low. The reader looks at the state of CTS prior to sending each
character. When the receiver is willing to accept a message from
the reader it should raise the Clear To Send (CTS) line. The
reader will resume sending messages, continuing where it left off
when the CTS line was lowered.
XON/XOFF Protocol
This option uses only the transmit/receive lines between the
reader and the receiver.
When the receiver wants to suspend transmission, it will send
the XOFF character (ASCII 19 decimal) to the reader. When the
receiver wants to resume transmission, it will send the XON
character (ASCII 17 decimal) to the reader. The reader will
resume sending messages, continuing where it left off when the
XOFF character was sent.