1.5 Mbps is the most commonly used transmission speed (76% of our inspected applications). This is the default speed when you create a new project in most PLC configuration tools.
Many users don't know what it comprehends and do not alter this value. Our experience is that several installations are running on a speed that is much too high for the application. The I/O and the PLC software is unable to keep up with the high speed cycle time. The data transfer might be fast, but the data is not updating.
It can even be taken a step further if you look at drives; a setpoint will have affect after considerable time (can be more than 100 ms). A network with 30 devices running at 1.5 Mbps has an average cycle time below 10 ms. In these cases it’s better to reduce the transmission speed. When the transmission speed is lower, you can install longer trunk cables, longer spur lines and the data is less vulnerable to electronic disturbances (the bits are wider)