Hi Bogey,

The A/B lines do not need to be pulled high, although sometimes users will place pull-up/pull-down resistors on those lines in order to provide a bias level when the bus is idle. You can read more about that here: www.ti.com/.../slyt324.pdf. The component values for Figure 29 can mostly be found in section 12.1 of the datasheet. The series resistors on the A and B lines are not defined there, but in general it is good to keep these values to 10 Ohms or less to limit attenuation of the RS-485 signal.

Under normal operation the A/B lines most likely will not reach +12 V, but this is defined as the limit of operation in case an application requires it. One example where this could occur would be if the node is receiving data from some other transceiver that is referenced to a different ground. In that case, ground differences between the nodes would be superimposed on top of the signal and could cause the voltages to exceed the power/ground rails of the device.

Please let me know if you have any other questions.

Regards,
Max