Have you ever used a safety network and ran out of room? There are only a certain number of allowed addresses on each safety network, and once you reach this number you have to start a new safety network. When it comes to AS-Interface, Safety at Work, the limit is 31 safety devices per network and 62 safety devices per gateway/safety monitor. This may seem like a lot, but the addresses go quickly.
AS-Interface is an industrial network that allows not only safety devices on its network but standard IO, too. Most people who use AS-Interface use it for both. This makes sense. Why run two networks when you have an emergency stop right next to an IO module? May as well connect them both to the same AS-i network.
AS-Interface does support extended addressing which allows up to 62 nodes per network, but this is only for standard IO modules and not safety devices. Digital IO modules can have up to 4 in and 4 out per module and can be addressed 1-31A or 1-31B for a total of 62. Safety modules, on the other hand, can be addresses 1-31 only. For every one safety device you put on the network, you remove the possibility of that address being used in an extended addressing fashion. For example, if address 1 is being used for a light curtain, then only addresses 2-31A and 2-31B, 60 total addresses, remain for digital and analog IO modules. AS-Interface was designed for eight IO per module such that the modules can be distributed completely around the machine. This relatively low IO count per module means the addresses get used up pretty quickly.
Let’s get back to the original question. What happens when you run out of room on your AS-Interface gateway and you still have more safety devices to attach to the network? You have to add another gateway, of course, and start running additional AS-Interface networks. You will have potentially four separate AS-i networks spread out over two AS-Interface gateways. (Gateways offer the possibility to connect up to two networks each.) Now what you have are multiple gateways with independent safety networks. If all the networks are truly independent, then you are done, no problem, but if a safety device must shut down a safety relay on another or both gateways, then you have a problem. The trouble is, how do you communicate between the gateways safely? The answer is safelink.
What is safelink?
Safelink is a safe Ethernet network that connects AS-Interface gateways and safety monitors. Using the ASIMON safety software, you assign safety devices to safelink safe bits. These safe bits can now be used by any one of the other 30 gateways out there on the safelink network. The safety data is broadcast traffic such that all gateways on the network can see the data all at the same time. It is an extremely fast and efficient way to transmit safety data. See Figure 1.
Figure 1: Safelink connecting multiple gateways
31 safelink devices
31 safelink bits per safelink device
The total number of safety devices that can be networked together is 62 safety modules per AS-Interface gateway. There are also up to 31 gateways in a safelink network. A total of 1922 safety devices can be networked together and can control a single release circuit. This is an incredible amount of safety data on one safety network. I am not sure it will ever be fully utilized, but the limits are large so you never have to worry about running out of room.
One limitation, and I am not sure it is one, is that only 31 safe bits can be transferred from a gateway to the safelink network. If you have 62 safety nodes on that gateway, then you would think there is a disparity. How can I transmit the state of 62 devices over 31 bits? The answer is pretty simple—group them together. You really only need to transfer a bit of safety devices in the same group. So if you have 20 e-stops in one safety group and 30 door switches in a second group, you really only need to send two safelink bits over the network. The groups are easily configured using the ASIMON software.
You can also group gateways on the safelink network. There can be up to 31 groups. If you decide to use groups, and multiple groups occupy the safe Ethernet switch, make sure to use a managed switch with IGMP snooping. The switch will then automatically separate the traffic and prevent one group’s traffic from being seen by participants in the other group.
Once your safelink network is set up and running, diagnostics is easy. The ASIMON software has a safelink diagnostics mode. Once turned on, you can easily see which safelink devices are talking and which are not. In Figure 2, you see three safelink devices at addresses 1, 2, and 3. The green color shows that communication is good between 1 and 2. The arrows also show the direction of safelink traffic. In this case, nodes 1 and 2 share data bidirectionally. Node 3 is on the safelink network but is not accepting safelink data and is not producing any. In this case, the color is red. The reason for the error is simple. I just haven’t configured this device to produce or consume data yet on the safelink network. The intuitive color codes and direction arrows make troubleshooting a safelink system easy.
Figure 2: Safelink diagnostics screen
Safelink compatible devices
Any of these compatible safelink devices can be combined to make your safelink network. You are not limited to using only one type. For example, you can use one EtherNet/IP gateway and then network that gateway with 30 other stand-alone safety monitors over safelink.