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HP 27130B
Host ENQ/ ACK Handshake. This option is used to pace the data transfer from the card to the device
to prevent the device from losing any data due to its slow internal processing speed.
The firmware sends an ENQ character after the pacing counter has counted down to zero. The card
then waits for an ACK character before proceeding to transmit more characters from the transmit
buffer. This will ensure that buffer space in the device is available,
You can program the pacing counter by using the Write Card Configuration, Subfunction 25 (WCC,
SF 25). The default count is 8
a
bytes. The counter is decremented after each character is transmitted.
There is a programmable handshake timer to prevent the firmware from being hung either if the
ACK is lost or if the device is off line and then comes on -line. The card will transmit an ENQ again
until an ACK is received. You have the option of disabling the ENQ retry after the time-out by set-
ting the "send message after ENQ timer time-out" bit in the Write Card Configuration, Subfunction 9
(WCC, SF 9). In this case the card will proceed with the data transmission from where it stopped.
If enabled, a handshake timeout will cause an event to be generated to the host.
The host ENQ/ ACK timer is programmed by using the WCC, SF 18. The default value used is 5
seconds.
NOTE
DO NOT TOGGLE CHARACTER HANDSHAKES TO
OFF
WHILE
WRITE
DA T A
ARE
BEING
TRANSMITTED. IF THE TRANSMITTER INITIATES
AN
ENQ/ ACK
HANDSHAKE
DURING
THE
DURA TION OF THE READ, THE ACK CHARACTER
FROM THE DEVICE WILL BE TAKEN AS DATA
(PLACED
IN THE
DATA
BUFFER)
AND
THE
TRANSMITTER
WILL
BE
HUNG,
PENDING
A
HANDSHAKE TIMEOUT IF ONE IS ENABLED. IF
THE DEVICE SENDS AN X -OFF CHARACTER TO
STOP THE TRANSMITTER, THIS TOO WILL BE
PLACED
IN
THE
DATA
BUFFER
AND
THE
TRANSMITTER WILL
NOT
STOP. THE TX BUFFER
EMPTY EVENT CAN BE USED TO ENSURE THAT A
TRANSMIT OPERATION IS NOT TAKING PLACE.
If the transmitter does become "stuck", the lire start output" request (Control Card, Subfunction 5) can
be used to continue transmitter operation. The handshake character, however, will remain in the
receive buffer.
Device
X -ON/X
-OFF Handshake. This handshake protocol allows the device to pace the
~ata
trans-
fer from the card to the device. The device will signal the card to stop transmitting data by sending
an X -OFF character. The receiving device restarts data transmission by sending the X-ON character,
or by sending any character if the Implicit Device X -ON option is enabled with WCC, SF 31.
An implicit device X -ON allows any received character to restart a transmission which was suspended
because of a received X -OFF. This is useful when communicating with a terminal where the user
may press X -OFF to suspend output.
4-7
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