Modem control register – Rainbow Electronics AT75C220 User Manual
Page 97
AT75C220
97
Modem Control Register
Register Name:US_MC
Access Type:Write-only
Reset Value: Undefined
This register controls the interface with the modem or data set (or a peripheral device emulating a modem). The contents of
the Control Register are indicated below.
•
DTR: Data Terminal Ready
This bit controls the NDTR output. When bit 0 is set to a logic 1, the NDTR output is forced to a logic 0.
When bit 0 is reset to a logic 0, the NDTR output is forced to a logic 1.
Note:
The NDTR output of the UART can be applied to an EIA inverting line driver to obtain proper polarity input at the succeeding
modem or data set.
•
RTS: Request to Send
This bit controls the NRTS output. Bit 1 affects the NRTS output in a manner identical to that described above for bit 0.
•
FCM: Flow Control Mode
When FCM is set high, the hardware can perform operations automatically depending on the state of NCTS and char-
acter transmission logic. Such changes take place immediately and are reflected in the values read in the Modem
Status Register. This flag is set low at reset.
In flow control mode, transmission should occur only if NCTS is active.
31
30
29
28
27
26
25
24
–
–
–
–
–
–
–
–
23
22
21
20
19
18
17
16
–
–
–
–
–
–
–
–
15
14
13
12
11
10
9
8
–
–
–
–
–
–
–
–
7
6
5
4
3
2
1
0
–
–
FCM
–
–
–
RTS
DTR