1. PC Modem Control
The 8250 UART supplied with the PC supports a limited number of RS-232-C modem functions:
The UART can be programmed to interrupt the PC when modem status changes occur.
- The ability to control modem conditions (through Modem
Control Register).
- The ability to determine the modem’s status (from the Modem
Status Register).

2. UART Port Addresses and Functions
Port Port Address Offset Uses
0x3F x2F Base address Transmission Register Buffer
Receive Register Buffer
Line Speed (LSB)
0x3F x2F Base address Interrupt Enable
Line Speed (MSB)
0x3FA x2FA Base address Interrupt Identification Register
0x3FB x2FB Base address Line Control Register
0x3FC x2FC Base address Modem Control Register
0x3FD x2FD Base address Line Status Register
0x3FE x2FE Base address Modem Status Register

3. Modem Control Register
DTR: data terminal ready
RTS: request to send
Out1: not used
Out2: enable UART interrupt
Loop: enter loopback mode
RTS
Out1
Out2
Loop
DTR

4. MCR Example #define MCR 0x3FC #define DTR 0x01 #define RTS 0x02
#define OUT x08
- To signal the modem that the PC is connected to the channel and has
data to send:
outportb(MCR, DTR+RTS+OUT2);
- To clear the modem:
outportb(MCR, OUT2);

5. Modem Status Register
- CTS: There has been a change in the Clear to Send signal
since the last time the MSR was read.
- CTS: The value of the Clear to Send signal
DSR RI CD
CTS

6. Modem Interrupts
- UART generates 4 types of interrupts (interrupt enable
register).
- Modem status changes can be made to cause interrupts
by setting IER to 0x08.
#define IER x3F9
#define DATA_AV x01
#define TX_HR_MT x02
#define RVC_LS x04
#define MDM_CHG x08
outportb(IER, MDM_CHG),
Data available
Tx holding reg. empty
Receive Line Status
Modem status change

7. Interrupt identification register
Modem Interrupts
- Interrupt identification register indicates the cause
of the interrupt.
IIR Interrupt
Receive line status (overrun, parity error, etc.)
Received data available
TX holding Reg. empty
Modem status change
Interrupt identification register
Interrupt pending
Interrupt id (b0)
Interrupt id (b1)
- The status of modem can be
obtained by reading the MSR
status = inportb(MSR);

8. Example: Commkit External Modem
- Extends the point-to-point utility to handle modems.
- The following modem status changes are recognized:
CTS, DSR, RI, RLSD (CD)
- Once carrier is detected, the modem enters the DSR state
and signals both DSR and RLSD to the UART.
- Upon detection of DSR and DLSD, the DTE responds with
DTR and RTS.
- At this point, DTE is connected DCE and communications
can commence.

9. Implementation
- Three states: WAIT_FOR_CARREIR, CONNECTED, DISCONECTING
- When a modem status change occurs, control is passed to
low_level( ), which sends modem status to Application.
- Foreground process controls the modem or sends messages
to serial port via low_level( ) too.
- Create two new process identifiers:
MODEM_DATA --> SP1IH (send data)
MODEM_CHANGE --> SP2IH (control modem)
- MODEM_DATA + code=MODEMSTATUS --> AP
AP --> MODEM_CHANGE
both cause control to pass to modem_change( )

10. Implementation KEYIH do_modem( ) keyboard_data() do_rmt_scr() XMITDONE
MODEM_DATA
(SP1IH)
do_rmt_scr()
MODEM_CHANGE
(SP2IH)

11. Hayes’ Commands
- Not all UART Manufacturers conform to RS-232-C signals.
- Programs may not be portable across different modems.
- Many modems support Hayes’ commands, a set of textual
strings that are recognized and interpreted by the modem.
- Hayes modem has a front-end that interprets the modem
commands issued by the application software, converting
them into the signals required by the modem.
- When first initialized, the front-end is in “command” state,
interpreting any data supplied to it.
- Once a connection is established with another modem, the
front-end enters the “on-line” state, passing data directly to
modem.

12. Hayes’ Commands - +++ : from “on-line” to “command”.
- AT: command prefix
- DTxxxx: dial phone number xxxx.
- H : hang up
- O : from “command” to “on-line”
- &Zn=x: assign a phone number, x, to an internal register n.
- S=n: dial phone number stored in register n.

13. Hayes Modem State Diagram
All Keystrokes
To Internal Modem
Command State
Successful Call Out,
Successful Call In,
ATO
Connect Message
+++
OK Message
On-Line State
All Keystrokes
To Remote

14. Example: Commkit Internal Modem
- Internal Modem is installed as a card in PC.
- Most internal modems support Hayes’ AT commands.
- For a Hayes modem,
- The control software is not in direct communication with
UART; rather, all modem control signals are via the AT
Commands.
- All modem status changes are interpreted by the internal
modem and returned to the control software as text via
the UART’s data-available interrupt.