Bsmce04u-pp | Manual

: If Mach3 is stuck in "Reset" mode, check the polarity (Active Low/High) of your E-Stop input in Config > Ports and Pins > Input Signals . Mach3 USB Motion Card (BSMCEO4U-PP) Installation Manual

(Manual Table 8-3):

The BSMCE04U-PP uses three distinct, isolated ground terminals to prevent noise and electrical issues: bsmce04u-pp manual