Soft ware Hand shak ing Using XON/XOFF
Basic operation and timing using XON/XOFF software handshaking in a trunking system is 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 RFCTS has al ready been as signed (the chan nel is avail able), it as -
serts its PTT to power up the ra dio’s trans mit ter. If the chan nel has not been as signed
yet, it sends XOFF to the send ing com puter and as serts PTT to ob tain the chan nel.
When the chan nel gets as signed, it sends XON to the com puter and con tin ues.
–pres ents tones onto the chan nel.
–sets its RTS/CTS de lay timer.
•The re ceiv ing mo dem 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.
•Af ter the RTS/CTS de lay timer has ex pired, data is trans mit ted.
•When ever the mo dem re ceives a char ac ter from the send ing de vice or trans mits any buf -
fered char ac ter, it re starts the PTT drop de lay timer.
•Once the mo dem stops re ceiv ing char ac ters and the PTT drop de lay timer has ex pired, the
mo dem re leases the ra dio trans mit ter 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 immediately to an XOFF command.
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