One form of PROFINET communication is data exchange via I/O addresses. This form of communication is possible in principle in parallel to communications via UDP and TCP connections. The STEP 7 configuration ensures that the bandwidth of the Industrial Ethernet network is loaded only up to 50% with data exchange via I/O addresses. The Industrial Ethernet CPs of S7-300 and S7-400 and PC stations support communication via UDP and TCP connections. For example, any TCP and UDP messages can be sent with the Industrial Ethernet CPs of S7-300 and S7-400 and the communication blocks FC5/50 "AG_SEND/AG_LSEND" and FC6/60 "AG_RCV/AG_LRCV". An individual node (Unicast MAC address) or a group of nodes (Multicast MAC address) can be addressed as target address. You can only use Multicast via UDP connections with the IE CPs of S7-300 and S7-400. If the UDP messages are sent to a group of nodes, then sender and recipients must be configured for receiving Multicast messages.
When transferring Multicast messages from the sender to the recipient, the following interaction occurs with data exchange via I/O addresses.
Messages are transferred by the Industrial Ethernet switches with the following procedures:
- Cut Through
- Store and Forward
The one used depends on the Industrial Ethernet switch implemented. All ERTEC-based devices (e.g. SCALANCE X 200IRT, CP443-1, ET200 I/O) use the Cut Through procedure. In this case, the Multicast messages are forwarded using the Cut Through procedure.
The Industrial Ethernet switch switches the Store and Forward if a Broadcast message or Multicast message is received locally by the module. This delays data transfer of the next message. The delay depends on the length of the message.
The table below shows the delays at a baud rate of 100 Mbit.
|Length of message
||Store and Forward
||Store and Forward
The throughput time of a Multicast message can be between 7µsec and 123µsec due to switchover from the Cut Through procedure to the Store and Forward procedure.
Communication via I/O addresses is run cyclically with a configured SendClock (for example, 1msec). The communication is monitored with a watchdog factor (typically 3*SendClock). If the message is delayed by this time because of the transfer, a failure of communication via I/O addresses is signalized.
- Cycle time for data exchange via I/O addresses: 250µsec.
- Watchdog factor: 3
- All CPUs use Multicast messages via the CP443-1.
- Multicast messages of 1518 bytes are transferred via UDP connections.
Fig. 01: Sample configuration
Each Multicast message delays data exchange via I/O addresses to the ET200 M by 120µsec in each switch. The total delay with 10 devices is thus 1.2msec. The ET200 M monitors the data exchange via I/O addresses at 740µs.
- Increase the cycle time for the data exchange via I/O addresses in STEP 7.
- Increase the watchdog factor in STEP 7.
- Use the reduction ratio in STEP 7.
- Make sure that the data volume of the Multicast messages is < 100 byte.
- Structure the wiring such that the Multicast messages via UDP connections collide only slightly or not at all with the data exchange via I/O addresses. For this you use the alternative ports or separate switches.
Fig. 02: Optimized configuration