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Link Interfaces Reference
3.14
— IOLINK-PRO & 520 Reference Manual
WAN Link Control-Signal Operation
CTS flow control is not supported.
It is assumed that Dial In/Dial Out modes of dial-up operation will be configured externally to the
bridge/router, i.e. on the attached modem, by setting Originate Only/Answer Only modes. A dataset or
DSU/CSU can be used for a leased-line operation.
1) When a call is made to the bridge/router, RI will be asserted by the modem. The bridge/router
responds by driving DTR and RTS high to signal to the partner’s modem that it is ready to establish
communications. The bridge/router then waits (for the duration of the CD Wait Time, default 60
seconds) for the partner’s modem to respond with incoming CD and DSR signals driven high. (With a
dataset or DSU/CSU, RI is not applicable and the bridge/router will respond when CD is high).
2) After DTR goes high, if the incoming CD is not detected within the CD Wait Time then DTR and
RTS are lowered for 5 seconds. This clears the link, hangs up the modem and causes the modem to
redial in an attempt to establish the link. (This “DTR toggle” polling is continuous when the link is
inactive. When DTR is low, and RI is subsequently asserted, then the bridge/router immediately
drives DTR and RTS high and again waits for the duration of CD Wait Time for CD to be asserted).
NOTE: If the level 2 process times out, DTR toggle will be started.
3) When incoming CD and DSR signals both go high, then it indicates that the local modem, etc. has
established communications with the remote modem and that communications may begin.
4) If the link is up, and CD is subsequently interrupted for a period of 10 seconds, then the bridge/router
assumes the signal is lost and will display an Alarm. As stated previously, if the level 2 protocols can
continue to send, they will be allowed to do so. This prevents minor line glitches from taking the link
down upon the loss of CD.
5) If the RI signal is asserted while DTR is already high, the CD Wait Timer is restarted. Once the CD
signal is received by the called modem, the state of the RI signal is unimportant.
6) In the Conditional Link mode, the secondary link will be inactive by holding DTR and RTS low (the
modem, etc., will normally be configured to drop the call when DTR and RTS are held low). When
the secondary link needs to become active, DTR and RTS will be raised and the modem will make the
call.