A CAN bus line should have two terminators 120 Ohm resistors in the ends of the line. 

Our GPRS / GPRS2 / WiFi / mRDU / 4G  units have an on-board 120 ohm terminator resistor. If there is no other unit connected to the tachograph then the tachographs C7 and C8 pin should be connected so the internal terminator resistor is enabled in the tachograph, by default our cables come with this strap.

If there is another unit connected to the CAN bus then the wire between the tachographs C7 and C8 lines should be cut.

Please check out our GPRS2 Connection scenarios or 4G connection scenarios

After the RDU is installed measure the CAN resistance between the CAN high (C5) and CAN low (C7) should be around 60 ohm.