Hi,
As Jesper pointed out, lower speed is more robust for external noise and cable wiring mistakes.
I personally always use 1.5 Mb speed in our plant and take much care for quality of wiring.
1- Use high quality connectors ( I use Siemens with screw connections internally, but there are many more good brands)
2- Use the right cable ( for me the pink cable with the green/red wires)
3- Dubble check wiring (A-wire = green B-wire = red) in all connectors, and check the connection of the shielding.
4- Dubble check only first and last connector has termination resistors ON.
( check witch on of the 2 incoming wire connections has the internal resistors connected, the Siemens connector has them connected to the A1 and B1 terminal screws, so the first and last connector must have the Profibus wires connected to this A1 and B1 connections ! ).
5- And last but not least, at this speed the minimum required length of 1 meter.
For the choice of 1,5 Mb that I have, because lots of equipment has default at 1.5 Mb, for Simocode modules it is the max speed possible and with a scanrate around 30 msec with maximum amount of data possible on the bus it is suitable for most applications.
Greetings
John.