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HP 27130B
The MUX firmware will stop data transmission as soon as the X -OFF character is received; however,
up to two characters may be transmitted before the stoppage because of the SIO FIFO buffer.
If the handshake is disabled while o~tput is in progress, a Restart Transmitter request may be needed
to prevent outbound data from getting stuck on the card.
The handshake timer is programmed using WCC, SF 18. If this timer should timeout, an event, if en-
abled' will be generated to inform the host. Data transnlission may be resumed with Control Card, SF
5.
Host X -ON/X -OFF Handshake. This handshake protocol allows the MUX to pace the data transfer
from the device to the MUX. The rv1UX sends the X -OFF character (DC 3 or CNTL -s) to the device to
stop data transmission when there is less than about 71 bytes of space in the receive buffer. The MUX
sends the X -ON character (DC 1 or CNTL-q) when buffer space becomes available again.
If the device continues to transmit da ta to the MUX after the MUX has sent an X -OFF, the data will
be added to the receive buffer until it overflows. Once the data buffer overflows, data will be lost. If
echoing is enabled, the received data will be echoed even after the X -OFF, but not after an overflow.
To prevent any deadlock situation, the MUX will transmit the X -ON character even if the device had
sent an X -OFF character as part of the device X -ON/X -OFF handshake.
Single Text Termination
For single text termination, you have the option of determining which characters are to be used as the
text terminator. An example of a single text terminator is the carriage-return character (CR, ODH).
The single text terminator is not added to the buffer unless enabled by WCC, SF 8. The character is
returned in the event status block and the request status block.
The Single Text Termination option is enabled by setting the End On Single Text Terminator option in
the read device data subfunction code or in the data block of the \VCC, SF 1. This option may also be
toggled (on/off) for the duration of the read device data request by setting the "toggle STT" bit in the
request subfunction.
The single text terminator characters are programmed by specifying the actual ASCII characters by
using the WCC, SF 32. A maximum of eight characters per port may be designated as text terminators
at any time.
End-On-Count
Text
Termination
The End-On-Count Text Termination option is enabled by setting the End-On-Count option in the
read device data subfunction code or in the data block of the WCC, SF 1. The count is programmed
by using the WCC, SF 2. When the count decrements to zero, the current receive record will be ter-
minated with the message type indicating an end-on-count.
The End -On -Count option should not be confused with the internal card end -on -count which is set
by the MUX firmware to 252 bytes. This internal count is used to manage the receive buffers on the
card. When this count is exhausted, the current record will be terminated and will be made available
to the host. The termination type will be set to "message terminated by the card, more data coming".
This procedure allows the host to start reading data from the card while the card is still receiving data
from the device.
4-8
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