Yes MN300, it did, but I will go through the entire diagnostics again today to double check all parameters.

Thanks for the utube TPS, viewed it and will do that too.

Question: what determines that a motor should run in one direction and not the other when an appropriate voltage is applied to it.

Also, if the DIR input voltage is close to zero both at rest and when the motor runs in a direction, what other factor determines that the motor should actually run and not be at rest.

Ultimately the direction is determined by the state of the direction pin on the TB6600 IC in the motor driver. That depends on the the amount of current flowing through it's input opto coupler from the BOB. The voltage that drives that current can be measured by connecting the positive lead of your meter to the PC 5V terminal and the negative lead to the relevant 'P' terminal.

When I asked about ever running the wrong direction when DIR is high I was thinking about problems related to the parallel port. Does it ever run the wrong way when DIR is low?

Yes, yesterday when I checked the DIR, direction also changes when the DIR is low. i.e. when activating the Y- command, the DIR voltage was low and remained low even though the motor changed direction. I will go through all of the diagnostics again though. Have just returned home after shopping and picking up the other CPU.

Can any one tell me in detail what the Mach3 and CPU software/hardware processes are between activating the jog command and the associated Voltage / current changes in the HC245 pins. There must be a number of settings and other variables within Mach3 and the CPU that have an effect; and that could conflict with each other. I must say that my understanding of what occurs within the software and CPU hardware is quite dim.

the only setting's witch influence the state of the DIR pin are the port and pin Settings in Motor Output TAB.

interesting would also be the state (voltage) of DIR+ DIR- directly measured at he TB.

Thanks TPS, the replacement PC was not functional, hard drive issues. I'm borrowing another tomorrow loaded with XP.

Did quite some diagnostics with the motors at rest; all within normal limits. Will have help tomorrow to measure current and voltage at the HC245, BOB and TB6600s, with the motors activated.

I have the Motor Output values entered as per the BOB manual and will measure the DIR values at both the BOB and TB6600s with the motors at rest and activated

The only way the stepper driver can randomly run the wrong way is if the stepper driver sees the DIR input as being in the wrong state. This could be from the signal being on the threshold between states or if there are pulses on the DIR line you can't see with a meter. A friend with an oscilloscope would be a big help in diagnosing the problem.

Earlier I asked about the high voltage level at the parallel port input and the low level at the 'P' terminals (inputs to the stepper driver). These are the conditions where the logic levels are most likely to be at their threshold. The port inputs would affect one direction and the 'P' terminals the other. Since both directions sometime fail it would take two types of failure to cause your problem. That's possible but not too likely.
Having stray pulses (or noise) on the DIR line wouldn't normally be too high on the list of probable causes but the fault has evaded capture so far.

Sometimes troubleshooting by replacing parts is easier than tracking down and measuring the exact mode of failure. We will learn more when you try another PC.

Problem solved , the Y TB6600 B- terminal was not making contact with the motor cable so the motor was activated by one coil only.
Such a simple situation. I soldered the wire end and folded it back over the insulation, inserted it into the terminal receptacle and tightened the screw. The screw pushed against the insulation and pushed the wire into the plastic corner of the terminal, so no contact. The torsion on that wire was such that every time I took it out of the terminal receptacle and then put it back again, it would enter in the same position 

Gentlemen, thanks for all your help. I'll put the machine back together again and prepare for the next phase - learning how to make something.

Have a great 2021 with good health

I'm surprised the motor ran at all, it couldn't have had much torque. Now you can get on with some serious motor setup.

Aaah so I'm not finished yet, OK, I'll put the motors and gantries back together again to prepare for that.

What should I be looking at to tune the motors