Spurs
Ideally no spur should lead to the bus participants (node 1 ... node n) because reflections occur
depending on the total cable length and the time-related processes on the bus. To avoid system
errors, spurs to a bus participant (e.g. I/O module) should not exceed a certain length. 2 m spurs
(referred to 125 kbits/s) are considered to be uncritical. The sum of all spurs in the whole system
should not exceed 30 m. In special cases the cable lengths of the line and spurs must be calculated
exactly.
（《ifm System Manual ecomatmobile ClassicController (CR0032) V02》p59）
iFM控制器的说明书中提及：为了避免系统错误，支线不能超过一定的长度。而在125Kbits/s的环境下，2m的支线长度被认为没有问题。
但是应该注意：所以的支线长度不能够超过30m。