Modems - Computer Busses - page 249

ATDT 54321

Connection made

OK

Connect 9600

Computer Modem

+++

OK

Disconnection made

ATH

OK

Figure 15.3 Commands and responses when making a connection

The modem contains various status registers called the S-registers which store modem
settings. Table 15.3 lists some of these registers (Appendix C gives a complete listing). The
S0 register sets the number of rings that must occur before the modem answers an incoming
call. If it is set to zero (0) then the modem will not answer incoming calls. The S1 register
stores the number of incoming rings when the modem is rung. S2 stores the escape character,
normally this is set to the '+' character and the S3 register stores the character which defines
the end of a command, normally the CR character (13 decimal).

Table 15.3 Modem registers

Register Function Range ( typical default)

S0 Rings to auto-answer 0-255 rings (0 rings)

S1 Ring counter 0-255 rings (0 rings)

S2 Escape character (43)

S3 Carriage return character (13)

S6 Wait time for dial tone 2-255 s (2 s)

S7 Wait time for carrier 1-255 s (50 s)

S8 Pause time for automatic dialling 0-255 (2 s)

15.5 Modem set-ups

Figure 15.4 shows a sample window from the Microsoft Windows Terminal program (in
both Microsoft Windows 3. x and Windows 95/98). It shows the modem commands window.
In this case, it can be seen that when the modem dials a number the prefix to the number di-
alled is ' ATDT '. The hang-up command sequence is ' +++ ATH '. A sample dialling window is
shown in Figure 15.5. In this case, the number dialled is 9,123456789 . A ',' character repre-
sents a delay. The actual delay is determined by the value in the S8 register (see Table 15.3).
Typically, this value is about 2 seconds.

On many private switched telephone exchanges in the UK a ' 9 ' must prefix the number if