Modem status register – Rainbow Electronics AT75C220 User Manual
Page 98
AT75C220
98
Modem Status Register
Register Name:US_MS
Access Type:Read-only
Reset Value:Undefined
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 a
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
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
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
Bit 2 indicates that the NRI input to the chip has changed from a low to a high state.
•
DDCD: Delta Data Carrier Detect
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.
•
DSR: Data Set Ready
This bit is the complement of the Data Set Ready (NDSR) input.
•
RI: Ring Indicator
This bit is the complement of the Ring Indicator (NRI) input.
•
DCD: Data Carrier Detect
This bit is the complement of the Data Carrier Detect (NDCD) input.
•
FCMS: Flow Control Status
This bit indicates the value of the FCM in the US_MC.
31
30
29
28
27
26
25
24
–
–
–
–
–
–
–
–
23
22
21
20
19
18
17
16
–
–
–
–
–
–
–
–
15
14
13
12
11
10
9
8
–
–
–
–
–
–
–
FCMS
7
6
5
4
3
2
1
0
DCD
RI
DSR
CTS
DDCD
TERI
DDSR
DCTS