Legal Removal Request

Post: Hi. I'm having issues using the RS485 Pi with modbus. If I connect it to the bus with only the slave (EVCO EVK411M7; http://www.evco.it/en/15754-evk) connected, the slave doesn't respond to queries. If I add another RS485-USB dongle to the bus it starts working (the dongle has to be plugged in the USB port; if it is not plugged, the communication is not working). I've checked the communication with an oscyloscope and the only difference is, that the whole signal shifts for about 200mV. The whole signal is completely the same (same form, same noise etc.). The difference between high and low is (if I remmeber correctly) about +2,2V.The peculiar thing is (it might not be peculiar since this is the first time I'm working with RS485 or modbus) when the communication is not working, the low signal is 0V and the high is 2,2V. When I power the RS485-USB dongle, the signal shifts for about -200mV, so the low is -0,2V and high is +2V. Is the first situation (lo:0V, hi:+2,2V) by the book. In other words, conforming to the RS485 standard (I can't find anyting usefull and definitive on the internet)?Just for comparision, the signal the RS485-USB dongle is producing is lo:-2V, hi:+2V. Any help would be very appreciated. Kind regards,Edi