As an IoT engineer, you work for a company which manufactures high-precision machine parts. The technology is equipped with a number of PLCs which only communicate on Modbus TCP/RDU industrial protocols. 
Which is the best way to integrate these devices to your IoT solution?

Answer options:

A.Set up a transparent gateway with a Azure Modbus module and connect the devices to the gateway
B.Register the devices in the IoT Hub as devices and connect them directly, via the Modbus protocol.
C.Develop a custom module which communicates with the PLCs and deploy it to a translation edge gateway
D.Set up a translation gateway with a Modbus TCP module and connect the devices to the gateway

Correct Answer: D
Option A is incorrect because transparent gateways simply transmit messages from the leaf devices which have their own IoT Hub identities and connect using either MQTT or AMQP protocols. For scenarios where protocol translation is needed, a translation gateway is needed.
Option B is incorrect because devices can connect to IoT hub via three protocols: AMQP, MQTT and HTTPS. Devices using other protocols can be integrated via edge gateways.
Option C is incorrect because you don’t need to develop your own Modbus translation module. The pre-configured Modbus TCP module is available on the Azure Marketplace. You can simply add it to your solution.
Option D is CORRECT because if you want to connect devices that use Modbus TCP or RTU protocols to an IoT hub, an IoT Edge device as a gateway can be used. The gateway reads data from the Modbus devices, then forwards that data to the cloud using a supported protocol.
Diagram: 

References: 

https://docs.microsoft.com/en-us/azure/iot-edge/deploy-modbus-gateway?view=iotedge-2018-06
https://github.com/Azure/iot-edge-modbus
https://azuremarketplace.microsoft.com/en-us/marketplace/apps/microsoft_iot.edge-modbus?tab=overview
https://docs.microsoft.com/en-us/azure/iot-edge/iot-edge-as-gateway?view=iotedge-2018-06