This question was originally posted on DCIM Support by Yang Gao on 2018-01-24
Suppose there are 10 slave MODBUS devices off a common gateway device such as a power meter.
Which of the following methods that DCE uses to get data from each device?
1, DCE establishes TCP/IP communication with gateway and get data from all slave devices and then close communication
2, DCE establishes TCP/IP communication with gateway and get data from one slave device and then close the communication. Do the same thing for next slave device.
This answer was originally posted on DCIM Support by Steven Marchetti on 2018-01-24
Hi Yang Gao,
If you'll notice the differences in discovery between SNMP and Modbus, you'll see that SNMP can discover groups of devices while Modbus discoveries are for just a single unit. That discovery includes the user specifying the IP of the gateway, the slave address of the device, and the DDF to be used for the individual device.
This is done because SNMP devices have vendor and device specific information built in and every device and manufacturer uses different SNMP OIDs. In Modbus, every device can use the same Modbus registers which makes it impossible to discover devices en-mass.
Subsequent polls for any device is specific to that device. You can have comm with one device on a gateway and no comm on another (assuming a device issue rather than a gateway issue).
To answer your question directly, it's more like #2. I can't say however if there are multiple threads or each individual devices is queued up behind the first. That also would not be specific to a gateway/IP, that would just be the background process.
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