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HP 27130A
Software Handshake with the Device
Three software handshakes are available between the host and the device:
I. Host controlled ENQ/ ACK handshake
2. Device controlled X-ON/X-OFF handshake
3. Host controlled X -ON/X -OFF handshake
Besides enabling the individual software handshake defined above by using the WCC, SF 9, you
must also enable the Software Handshake option in WCC, SF
1.
Handshakes may be toggled on or
off for the duration of the request by setting the IItoggle handshake" bit in the Read Device
Data request (RQ
1)
subfunction field.
The handshakes will revert to the previous definition
w hen the request completes.
The characters defined for the ENQ, ACK, X-ON, and X-OFF characters are programmable by
using the WCC, SF 22, SF 23, and SF 24.
The following discussion will assume the default characters defined below.
handshake
default ASCII
hex
character
character
value
ENQ
ENQ
05
ACK
ACK
06
X-ON
' DC1
11
X-OFF
DC3
13
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 80 bytes. The counter is decremented after each character is
transmitted.
There is a programmable handshake timer to prevent the firmware from being hung 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 setting 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
wec,
SF 1 8. The default value used is 5
seconds.
4-7
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