Should you use a managed switch or an unmanaged switch with PROFINET? Yes, you should use either a managed or an unmanaged Industrial Ethernet switch with PROFINET ;). Both types of switches are suitable for PROFINET networks.
An unmanaged switch has no built-in intelligence except to send incoming Ethernet frames out the correct port – the port to which the destination device is connected. And only that port. For diagnostics, unmanaged switches have LEDs. Each port has an LED indicating that there is Ethernet traffic. However, they do not provide advanced information about bandwidth utilization or the number of retries, for example.
You can buy unmanaged switches from many vendors that meet the minimum requirements for PROFINET: a 100 Mbps full-duplex Industrial Ethernet switch. Also, the Quality of Service (QoS) feature is recommended, since PROFINET frames are set automatically with a priority higher than TCP/IP frames. If a TCP/IP frame and PROFINET frame arrive at the same time, the PROFINET frame gets sent out first.
A managed switch adds intelligence that an unmanaged switch does not have. Several IT protocols are useful with managed switches, including Simple Network Management Protocol (SNMP), Link Layer Discovery Protocol (LLDP), Internet Group Management Protocol (IGMP), and Hyper Text Transfer Protocol (HTTP). Let’s dispense with IGMP right off the bat – PROFINET does not need IGMP, because PROFINET unicasts.
SNMP and LLDP are specialized protocols for retrieving data that switches save. For example, to set up and receive alerts when there is an excessive number of retries, SNMP can help. If you want to read the bandwidth utilization is in real-time, SNMP can help. LLDP allows software to extract network topology information. LLDP also enables one unique feature of PROFINET: Simple Device Replacement. You can learn more in this MinutePROFINET video, and watch a demonstration in this video from our PROFINET one-day training classes. Finally, HTTP is there to support a switch’s web page. Many switches offer a web page where further diagnostics or configuration options can be easily accessed from your web browser of choice. Managed switches might also offer other additional features such as port mirroring for traffic capture and redundancy options.
“You will either use a managed switch… or wish you had.”
A managed switch provides information that can help prevent downtime and speed troubleshooting when the line does go down. Managed switches cost more upfront, but they are an investment. The incremental cost is trivial when compared to the cost of downtime.
Up to now, you probably think this topic covers standalone Ethernet switches. That is true. But managed switches are also contained within PROFINET devices. The same kind of diagnostic information you can extract from a standalone switch can be obtained from a switch in a PROFINET device. For example, you can use an OPC-SNMP server to read information from a managed switch and display it and alarm it in an HMI. Then, the PROFINET controller can take action as a result of diagnostic information.
An Ethernet switch can also be a PROFINET device. PROFINET switch devices look like an IO to the PLC. They have a GSD file, just like any other PROFINET device. And the GSD file defines the device diagnostic information. For example, if a device is unplugged from a switch port, the PLC logic can execute an appropriate action. The diagnostics information is even more valuable if the switch is a Media Redundancy Protocol (MRP) device. If the switch reports a failure in the ring, the PLC can alarm it and act on it. It can pinpoint where the break in the ring is located, simplifying the maintenance response. Many vendors supply Ethernet switches that can also act as PROFINET IO devices.
With PROFINET you can use an unmanaged switch, a managed switch, or a switch that is also a PROFINET device. It’s good to have options. But, even though you can, it isn’t always recommended to use an unmanaged switch –unless you can afford the downtime.