SIM900D Hardware Design
3.14 Behaviors of the RI
Table21: Behaviours of the RI
State
RI respond
Standby
HIGH
Voice calling
Change LOW, then:
（
1
）
Change to HIGH when the call is established.
（
2
）
Use AT command ATH, the RI pin will be changed to HIGH.
（
3
）
When sender hangs up, it will be changed to HIGH.
Data calling
Change LOW, then
：
（
1
）
Change to HIGH when the call is established.
（
2
）
Use AT command ATH, the RI will be changed to HIGH.
SMS
Change to low and hold low level for about 120 ms when the module receive a SMS, and then
it is changed to HIGH.
URC
Some URCs triggers 120ms low level on RI.
For more details, please refer to related
document [10]
If the module is used as caller, the RI will maintain high. However, when it is used as receiver, following is timing
of the ring.
Figure29: RI behaviour of voice calling as a receiver
Figure30: RI behaviour of data calling as a receiver
SIM900D_Hardware Design_V1.04
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2010.10.09
