Combine a Yocto-4-20mA-Tx with a YoctoHub-GSM-3G-EU to get an autonomous appliance with built-in network connectivity:
The Yocto-4-20mA-Tx is a USB 4-20mA signal generator. With this device you will be able to drive any other device featuring a 4-20mA input. This is an isolated device: the USB part is electrically isolated from the current loop part. The Yocto-4-20mA-Tx can drive any current loop powered from 12 to 24V with a 0.1% precision. Cherry on the cake, the device can detect if the loop is properly powered or not.
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The YoctoHub-GSM-3G-EU is a wireless-enabled module that can host three Yoctopuce modules to access them remotely through a 3G GSM cellular network (UMTS and HSPA). It can be powered either by a USB Micro-B cable and a regular phone charger, or a 5V battery pack.
The radio module supports the two GSM frequency bands 900 MHz and 1800 MHz used in Europa, Middle-East, Africa, Asia and Pacific. For a detailed list of frequency bands supported in each country, please refer to the Wikipedia page GSM_frequency_bands.