I have been looking at how the BOB board connects to the rest of the system and see a problem with the stepper drivers. The TB6600 stepper drivers have opto-isolator inputs. The input signal current is a minimum of 8 mA. This is far more current than a normal logic input would require. The signals from the parallel port are buffered by SN74HC245 Octal Bus Transceivers. The datasheet says the output current of that device is 7 mA. That’s not to say it can’t provide more, but the voltage out may be lower than the level that is guaranteed to work with the input of other logic ICs. While this may be marginal design my main concern is with the enable signals.

Unlike the other signals, the enable signal, Pin 14, connects to 3 buffers. There are five  4 pin connectors provided for connecting to the stepper drivers. X and Y are connected to one output, Y, A and B are connected to the second. The third output goes to the P14 terminal. If the 4 pin connectors are used as intended the output shared by X and Y will have to supply 16 mA. As there is no A or B, the Z axis doesn’t have to share.
A solution would be to connect the enable of either X or Y to the Pin 14 terminal.

Another alternative way to wire would be to use the P terminals. The step and direction signals, P2 to P9 plus P16 and P17, also appear on the terminal strip. The 3 stepper drivers could be wired to the terminal strip instead of the 4 pin connectors. The driver enables would connect to P14 and the enable pins of the X (or Y) 4 pin connector and the Z connector. I think the connectors advertised as DuPont connectors would fit onto single pins of the 4 pin connectors. Many hobby sources carry these. Here is an example.
https://www.ebay.com/itm/40pcs-Dupont-Male-to-Female-10cm-Jumper-Wire-Connectors-Lead/254793576268?hash=item3b52e16f4c:g:bPAAAOSwASBdJbQJ
This method also gives you a way to connect if the mating 4 pin plugs do not come with the BOB.

HC245 are very happy driving TB6600 drivers, I had 0 (zero) problems with them.
Don't look at the voltage levels, optocouplers in TB6600 are not logic gates. LED on the input side of the optocoupler is an "animal" that is fed by current, not voltage. Usually nominal forward voltage for these LEDs is around 1.2 - 1.3V.

Good tutorial, thanks
I'll let you two come to terms on the issues raised by MN300, in the mean time I'll read up on the componentry and processes mentioned.

If necessary, could always obtain a more suitable BOB.

I can't argue with your successful use of the BOB board. As I said, the IC specs define use in a logic system and the IC will put out more current if overloaded. It would be interesting to know the actual voltage being delivered to the opto input or better yet the current.
Hopefully the designer of the stepper driver left a safety margin between the minimum current that will just trigger the input and the minimum operating level in the spec. I don't have the information to decide if you are operating in the area between or are over the 8 mA minimum.

Once I have the items, will I be able to provide you with the voltage and current to the opto input?

So you are saying that the BOB IC (is this the SN74HC245 Octal Bus Transceiver?) needs to put out more current than it is rated for, to bridge the communication gap of the TB6600 opto coupler. So if I connected them as ZASto does, what is the worst that could happen, BOB blows up or TB6600 blows up ....... or both??? What damage could occur if the BOB IC was overloaded??

Do either of you have circuit diagrams of both the BOB and the TB6600? If Yes, could you attach them? I'd like to follow the processes.

Better information may come with the items but I wouldn't count on it.

If damage were to occur it would be to the SN74HC245. Another possible problem would be if the opto-coupler is just barely being turned on. Then it would be more susceptible to noise and could be the cause of missing steps. Since there is an unused buffer why not use it?

ZASto has provided the information for the BOB in a previous post.
https://www.machsupport.com/forum/index.php?topic=43939.msg282588#msg282588

Here is what I found for the TB6600.
https://www.mcielectronics.cl/website_MCI/static/documents/TB6600_data_sheet.pdf

Have you planned a clean place with good ventilation to mount the new equipment?

Can the voltage and current be measured with my you beaut QM1529 Digitech multimeter?

Since I won't have encoders attached I'd like to reduce the chance of missing steps in every way.
How would you see the TB6600s hooked up to the BOB, your before mentioned 1st or 2nd alternative?
If afterwards it somehow caused an issue, I can always connect the units as ZASto did

Suitable equipment location is available

Well, I use this BOB for years without any glitch.
We can widen this discussion on "are parallel port electronic components capable of driving opto couplers", but please don't do it.

My first BOB was without any buffering, LPT pins directly connected to optos in motor drivers and switches connected directly to LPT pins.
That was in early days me dealing with CNC.

Good to hear from you ZASto, I have no doubt that you have not experienced any glitches.
Being the adventurous type, I would not mind trying it as per MN300's suggestion, but any difficulties and I'll default your way.

From the BOB circuitry diagram you attached some time ago, it seems to my untrained eye that the BOB has 6 opto couplers and the diagram states that they are for pins 10, 11, 12, 13, 15 and the 6th for the spindle control maybe?? What is the significance of that as related to using pin 14?

Pin 14 is one of the buffered outputs, no different from the others except that it is used to control more than one device. Here is a description of the BOB that might be informative.

Pins 10, 11, 12, 13 and 15 are the inputs for the limits switches or whatever you chose to connect to them.  The switches are powered by a 10 volt supply regulated down from the 24 volt supply. Their connection to the parallel port inputs is isolated through 5 of the opto couplers.

The 6th opto coupler connects pin 1, an output from the parallel port, to the circuit that changes the PWM spindle signal to analog. In the BOB diagram just to the left of the 6th opto-isolator there are two triangle symbols. These the sections of the SN74HD245s that buffer pin 1. Those sections and the 10K resistor are also shown on the left side of the diagram as IC2 and the resistor attached to pin 1.

Other outputs from the parallel port are not opto-isolated by the BOB. They are buffered by the SN47HC245 ICs and connect to the terminal strip. Stepper signals also connect to the 4 pin connectors. The opto couplers in the drivers isolate them from the parallel port and the 5V supply.

Pin 17 can be jumpered to operate the relay. That circuit is on the middle right of the diagram. The resistor and SN74HC245 section are also shown on the left like the analog circuit. The relay common is shown connected to the 24 volt common. That seems a bit strange as relays are often used to isolate circuits.

The relay, the input switches and the analog output share the same common which is isolated from the parallel port, the buffered outputs and 5 volt common. The parallel port receives ground from the PC. The 24 volt common should be separately grounded for safety, especially if the connections to the limits switches are not protected.