Profibus DP RS485 Polarity
B - red wire, plus, DSUB9 pin3
A - green wire, minus, DSUB9 pin8

Maybe the cable is swapped on slave device. Do you use Siemens profibus connectors, if yes then it is almost impossible to wire them wrong, but I have seen also this scenario. Did you make the slave, if yes check that B is biased at Vcc and A is biased at gnd?
Here is one exaple where RS485 transceiver has AB swapped with respect to Siemens AB marking, so the plausible explanation is: Siemens vs Others = swapped. You know, in Simens averything is done a little bit complicated as it should be, if everywhere in the world use A and B, they use B and A just to make things more difficult.

http://www.nve.com/isoapps-PROFIBUS.php