The previous answers are all correct. Knowing the maximum, you have to consider multiple items:   1: Maximum supported DIO devices: HSBY CPU: 61 (max = 64, 3 are reserved for Ethernet/IP local slaves) BMENOC0321: 112 (max = 124, 12 are reserved for Ethernet/IP local slaves)   Be aware that connecting DIO devices direct to the device ports of a HSBY CPU might not work as the CPU device ports have a VLAN active, which might not be supported by the device. To overcome this issue, use external switches or use BMENOS0200 switch in the CPU racks.   2: Maximum supported input/output size HSBY CPU: 4Kbyte IN + 4Kbyte out (see previous answers) BMENOC0321: 4Kbyte IN + 4Kbyte out (see previous answers) Basically this means that the amount of data communicated is DIO device type dependent. You have to calculate the amount of data each device is communicating + take the overhead into consideration. I use to create some configuration and see what the data is used for each specific device type.   3: Maximum communication bandwith of an interface HSBY CPU: 15000 packets BMENOC0321: 5500 packets The amount of packets is depending on the configured RPI (Repeat Packet Interval / Repeat Rate / update rate). For example when you define 100 devices with an RPI of 10 mS, will result in minimal 2(Read+write) * (1000/10) * 100(devices) = 20.000 packets. Solution will be to decrease the RPI for some/all devices, to keep the amount of packets between the limits.   4: Physical network limitation When using a RSTP ring functionality, the limitation is to have max. 40 device within a ring. As  both CPU's count as a RSTP device, this limits the amount of DIO devices in a ring to 38. This imitation comes from fixed RSTP parameter settings in Schneider Electric DIO devices with RSTP functionality enabled (ie Modicon STB, Altivar Process). To overcome this limitation you can create a network star (using switches) or create multiple network rings (also by using switches)  ... View more