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Soft ware Hand shak ing Using XON/XOFF
Basic operation and timing using XON/XOFF software handshaking are as follows:
The send ing com puter be gins send ing data. The for mat must be 8 data bits, no par ity,
1 stop bit.
Upon de tect ing data, the send ing mo dem per forms the fol low ing:
–
checks the chan nel, if this fea ture is en abled. If the chan nel is free, it as serts its PTT to
power up the ra dio's trans mit ter. If the chan nel is busy, it re turns XOFF to the send ing
com puter, which halts the trans mis sion of data. When the chan nel be comes free, the
mo dem sends XON to the com puter, which re sumes send ing data. The mo dem buff ers
the data while it con tin ues with the fol low ing:
–
pres ents a mark tone (car rier) onto the chan nel.
–
sets its RTS/CTS de lay timer (from 20 to 300 msec). RTS/CTS de lay is a mis no mer in
this case as soft ware hand shak ing does not use RTS and CTS sig nals. It is ac tu ally a
buff er ing time be tween when the mo dem starts re ceiv ing data from the com puter and
when it starts trans mit ting. This de lay must be greater than the car rier de tect de lay at
the re ceiv ing end.
The mo dem at the re ceiv ing end senses car rier on the line and as serts its CD. The re ceived
data is muted for 5 or 50 msec to en sure that tran sients do not af fect the re ceived data.
At the send ing end, once the RTS/CTS de lay timer has ex pired, data is trans mit ted.
–
The send ing mo dem's PTT drop de lay timer is started at the end of the last trans mit ted
char ac ter.
Once the PTT drop de lay timer has ex pired, the mo dem re leases the ra dio trans mit ter, turns
off the mo dem car rier and re turns to re ceive mode.
This configuration can buffer 500 msec worth of characters. To prevent internal data buffer
overflow, it is important that the RTS/CTS delay timer be set to a value less than 500 msec and
that the computer respond to XOFF immediately.
For software handshaking, set XON/XOFF signaling on, assert RTS (continuous) off, and
buffering on. Do not connect hardware RTS and CTS. Set RTS/CTS delay and PTT drop delay
as required.
No Hand shak ing, Buffered
Basic operation and timing with data buffering (no handshaking) are as follows:
The send ing com puter be gins send ing data. Data for mat must be 8 bits, no par ity, 1 stop bit.
Upon de tect ing data, the send ing mo dem per forms the fol low ing:
B1474
BARNETT EN GI NEERING LTD.
PAGE 10
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