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4 OP ER A TION
The B1474 operates from 12 to 30 VDC to provide an interface between two computer devices
and the communications channel. In the descriptions below it is assumed that the sending
device uses a DCE configuration.
Four different types of operation may be configured:
Hard ware Hand shak ing
Basic operation and timing using hardware handshaking are as follows:
• The send ing de vice as serts its RTS and waits.
• Upon re ceiv ing the RTS, the as so ci ated mo dem per forms the fol low ing func tions:
– 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.
– pres ents a mark tone (car rier) onto the chan nel.
– sets its RTS/CTS de lay timer (from 20 to 1000 msec). RTS/CTS 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, the mo dem re turns a CTS to
the send ing com puter.
• Data is trans mit ted; any for mat may be used.
• Once all the data has been trans mit ted, the send ing com puter drops its RTS.
• Upon sens ing the dropped RTS, the send ing mo dem does the fol low ing:
– drops CTS im me di ately.
– sets its PTT drop de lay timer (0 to 250 msec).
• Once the PTT drop de lay timer has ex pired, the send ing mo dem re leases the ra dio trans mit -
ter, turns off the mo dem car rier and re turns to re ceive mode.
For hardware handshaking, set XON/XOFF signaling off, assert RTS (continuous) off and
buffering off. Set RTS/CTS delay and PTT drop delay as required.
Refer to Figure 1 for a summary of the modem timings.
B1474
BARNETT EN GI NEERING LTD.
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