if encoder wire broke, Can the rmc150 run in open loop?
Yes. The controller will halt once it loses feedback, but once it does you can send a Direct Output (9) command with the open loop voltage you want the RMC150 to output.
You can do this automatically by creating a program trigger on the No Transducer error bit that runs a program which sends the Direct Output command.
what’s the halt decel rate when it halts?
The halt would ramp the control output voltage down at the rate specified in the Open Loop Ramp Rate parameter in the axis tools on the all tab in the Halts section
If you use a program trigger, since it would send a command the next loop, the voltage would not have enough time to ramp to zero.