Sierra Wireless MP 875 - Manual (Page 45)

Configuring the MP Modem to Report GPS and I/O Data
A digital input can be connected to four of the pins on the
DB15HD connector: Pins 3, 4, 11, and 12. (Pins 3 and 11 could
alternatively be used for digital output.)
Note: Before using the input/output lines, you must configure them as
inputs or outputs.
Typically a digital input device should be connected between
Ground (Pin 10) and the input port (Pin 3, 4, 11, or 12).
Figure 4-2: A button wired to Pin 4 (digital input) and Pin 10 (Ground).
The pins report a logic high on an input of 3.45 VDC. An
internal pull-up resistor provides a high condition when the
switch is open.
The digital input pins report a logic low on an input between
0 VDC and 0.8 VDC. Sinking the input pin to ground yields a
logic low (0x00) when the port is polled.
If you configure the MP modem to send data to a network
server, digital input data can be remotely monitored.
Example: panic button connections
An I/O cable for a panic button requires a wire to one of the
digital input or I/O pins (such as #3) and one wire to the
Ground pin (#10).
Digital output devices
Digital output devices are those that have only two states and
the state is controlled by a signal from the MP modem. Any
device that is to be switched on and off from 3G Watcher
would be installed as a digital output device.
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