Manual

AT75C310

Modem Status Register

This register provides the current state of the control lines from the modem (or peripheral device) to the CPU. In addition to

this current-state information, four bits of the Modem Status Register provide change information. These bits are set to

logic 1 whenever a control input from the modem changes state. They are reset to logic 0 whenever the CPU reads the

Modem Status Register.

• DCTS: Delta Clear to Send

This bit is the Delta Clear to Send indicator. Bit 0 indicates that the NCTS input to the chip has changed state since the

last time it was read by the CPU.

• DDSR: Delta Data Set Ready

This bit is the Delta Data Set Ready indicator. Bit 1 indicates that the NDSR input to the chip has changed state since

the last time it was read by the CPU.

• TERI: Trailing Edge Ring Indicator

This bit is the Trailing Edge of Ring Indicator detector. Bit 2 indicates that the NRI input to the chip has changed from a

low to a high state.

• DDCD: Delta Data Carrier Detect

This bit is the Delta Data Carrier Detect indicator. Bit 3 indicates that the NDCD input has changed state.

Note that whenever bit 0, 1, 2, or 3 is set to logic 1, a Modem Status Interrupt is generated. This is reflected in the

modem status register.

• CTS: Clear to Send

This bit is the complement of the Clear to Send (NCTS) input. If bit 4 (loop) of the MCR is set to a 1, this bit is equiva-

lent to RTS in the MCR.

• DSR: Data Set Ready

This bit is the complement of the Data Set Ready (NDSR) input. If bit 4 (loop) of the MCR is set to a 1, this bit is equiv-

alent to DTR in the MCR.

• RI: Ring Indicator

This bit is the complement of the Ring Indicator (NRI) input. If bit 4 (loop) of the MCR is set to a 1, this bit is equivalent

to OUT1 in the MCR.

• DCD: Data Carrier Detect

This bit is the complement of the Data Carrier Detect (NDCD) input. If bit 4 (loop) of the MCR is set to a 1, this bit is

equivalent to OUT2 in the MCR.

• FCMS: Flow Control Status

This bit indicates the value of the FCM in the Modem Control Register.

31 30 29 28 27 26 25 24

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23 22 21 20 19 18 17 16

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15 14 13 12 11 10 9 8

–––––––FCMS

76543210

DCD RI DSR CTS DDCD TERI DDSR DCTS