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7. HSD COMMUNICATIONS
7.4
Receiving an HSD Call
When an HSD call is received the equipment normally answers it automatically.
Note: Some HSD terminals cannot receive.
The internal HSD interface
Direct call
The HSD Terminal uses the DTR signal to respond to received HSD call. DTR is
off when the equipment is idle. When a call arrives, the FELCOM 82 turns on RI.
The HSD Terminal detects the RI signal and turns on DTR. This causes the
FELCOM 82 to turn off RI and turn on DSR, CTS, DCD, to communicate over
land line.
Address call
DTR is not used in receiving an address call; the HSD Terminal uses it for
sending data, such as control commands, to the FELCOM 82. When an address
call arrives the HSD Terminal responds according to DTR state as below.
Address call and DTR ON
When an address call arrives, the FELCOM 82 turns off RI and sends the
indication "INC" to the HSD Terminal. The FELCOM 82 starts receiving
processing five seconds after receiving "INC." When processing is completed, RI
goes off and DSR, CTS, DCD go on, to communicate over land line.
The HSD Terminal may send the "DIC" command to the FELCOM 82 within five
seconds of receiving "INC," telling the FELCOM 82 to stop receiving processing.
This is done in the case of overlapping calls. If this occurs while is call is being
relayed, the CIC command is sent to the FELCOM 82 to reinitiate processing.
Address call and DTR OFF
When an address call arrives, the FELCOM 82 turns on RI. This tells the HSD
Terminal to turn on DTR, which causes the FELCOM 82 to send "INC" to the
HSD Terminal to indicate it is receiving. The HSD Terminal then sends the CIC
command to the FELCOM 82 to tell it to start receiving processing. When
processing is completed, RI goes off and DSR, CTS, DCD go on, to
communicate over land line.
When the HSD Terminal sends DIC instead of CIC, the FELCOM 82 sends the
indication VAL to stop receiving processing. Furthermore, when the CRN
command or other three-character command is sent the FELCOM 82 sends the
indication INV.
7-8
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