Serial Communications (16C452)
Ring Indicator Inputs (RI0*, RI1*)
When active (low), RI* indicates that a telephone ringing signal has
been received by the modem or data set. The RI* signal is a modem
control input whose condition is tested by reading the RI*, bit 6, of
the associated UART’s MSR. The MSR output bit TERI, bit 2,
indicates whether the RI* input has changed from high to low since
the previous reading of the same MSR. If the interrupt is enabled
(indicated by the Interrupt Enable register bit 3 being set to 1), and
RI* changes from high to low, an interrupt is generated for that
UART.
Receive Line Signal Detect (RLSD0*, RLSD1*)
When active (low), RLSD* (sometimes referred to as Data Carrier
Detect, or DCD) indicates that the data carrier has been detected by
the modem or data set. RLSD* is a modem input whose condition can
be tested by reading the RLSD bit of the Modem Status register
(MSR), bit 7. The DRLSD bit of the MSR, bit 3, indicates whether
the RLSD input has changed since the previous reading of the MSR.
RLSD* has no effect on the receiver. If the RLSD* changes state with
the modem status interrupt enabled, an interrupt is generated.
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