Most electronic circuits generate a Power On Reset with a circuit rather like this:
v + 5 Volt | | + --+-- --+-- 10 µF | - | *----> Power-On Reset | | +++ | | +++ 82 kΩ | | ^ Gnd
A disadvantage of this method is that it doesn't work when the mains is down
and then comes up again; The in rush current of the all equipment connected
to the mains will reduce the mains voltage, which in turn makes the supply
voltage of the equipment rise to slow for a proper reset.
By hard wiring the hardware reset to DTR one can reset the modem hardware
with a DTR toggle.
The RS232 cable uses +12 Volt for a logical `0' and -12 V for the `1'. Inside the modem however, 0 and 5 V are used. To translate these signal levels level translators are used. The `88' chip translates TTL (0 and 5 V) to 232, while the `89' IC does the opposite;
+ 13 12 11 10 9 8
+--O--O---O---O--O---O---O--+
| | |\| | | |\| | |
| +-| >O-+ +-| >O-+ |
| |/ |/ |
) |
| |\ |\ |
| +-| >O-+ +-| >O-+ |
|o | |/| | | |/| | |
+--O---O---O--O---O---O--O--+
1 2 3 4 5 6 Gnd
SN75189A
Quad RS232 level translator
My modem (Tornado 28k8 external) uses pin 13 for DTR:
|\
DTR >-----| >O----> -DTR
232 13 |/ 11 TTL
Adding the following circuit will reset the modem on the low to high transition of DTR;
+----*------*--< + 5 Volt
| | |
| | |
--+-- +++ 100 |
1N4148 /|\ | | kΩ |
--+-- +++ |
| | | |/
+----+ -| |+ | | |/ -
-DTR >--+ +--+ +--*----*--+ BC558B
+----+ | | |\
10 k 10 µF | \---> Reset
The next circuit resets on the high to low transition;
v + 5 Volt
|
10 µF |
| /
+----+ +| |- |/
-DTR >--+ +--+ +--*----*--+ BC548B
+----+ | | | | |\
10 kΩ | | | \|
--+-- +++ ----> Reset
1N4148 /|\ | |
--+-- +++ 100 kΩ
| |
| |
+----*------< Gnd
Any general purpose low power low frequent PNP Si transistor can be used instead of the BC558B. The BC548B is its NPN complement.
The usual disclaimers apply.