Table of Contents
Advertisement
HP 27130A
I
CAUTION
I
DO NOT TOGGLE CHARACTER HANDSHAKES TO
OFF
WHILE
WRITE
DA T A
IS
BEING
TRANSMITTED.
IF
THE
TRANSMITTER
INITIATES AN ENQ/ACK HANDSHAKE DURING
THE
DURATION
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 XOFF 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 "restart 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 data
transfer from the card to the device. The device will signal the card to stop transmitting data by send-
ing 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 XON allows any received character to restart a transmission which was suspended
because of a received XOFF. This is useful when communicating with a terminal where the user
may press XOFF to suspend output.
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 due to the SIO FIFO buffer.
If the handshake is disabled while output 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 1 8. If
this timer should timeout, an event, if en-
abled' will be generated to inform the host. Data transmission 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 MUX sends the X-OFF character (DC3 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 I or CNTL-q) when buffer space becomes available again.
If the device continues to transmit data 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.
4-8
Advertisement
Table of Contents
