Is 1.8V UART compatible with Beaglebone Black?
Typically CMOS uses 0.7 VDD to detect high level. At 3.3V this means 2.3V. The data coming from the UART might not be correctly read by the beaglebone.

Conversely, if your UART cable is not 3.3V tolerant, then you might damage the Uart and possibly the beaglebone.  

Level translators might be required.

If you want, you might use very simple level shifters:







simulate this circuit &ndash; Schematic created using CircuitLab

The working principle is simple. 

If CABLE_OUT is high, the MOSFET is off, R2 pulls up BEAGLE_BONE_RX high. Instead, if CABLE_OUT is low, the MOSFET is ON, and brings the LOW value to BEAGLE_BONE_RX side. 

The second circuit is just a 3.3 to 1.8V divider.

Note that the mosfet must be 1.8V logic level type!!! Furthermore none of the two  circuit is regenerative. Use short wires. Place the first one close to the beaglebone, the second one close to the cable, to reduce noise pickup.