Sounds like you may have an ‘Enable’ signal after all.
Assuming that the pin assignments on your LPT cable are ‘standard printer port’ then with the machine connected to your PC and Mach3 running change the ‘active state’ (hi / low), one at a time (and click Apply / OK after every change), for each output (1-9,14,16,17) to determine which pin and what state (hi / low) causes your motors to ‘lock’.
Tweakie.