This Old Mill

The journey of a guy getting an old CNC milling machine up and running in his garage…

Serial Connection

With the serial ports of this old mill and the computer (PC) properly configured, it’s time to make the physical connection between them.  To learn more about how the Allen Bradley 8400MP (AB control) communicates with a PC, visit my Connecting the Beast? page.

The following diagram illustrates the cable needed for this old mill to be able to talk with a PC and vise-versa.  I don’t know if colors of the lines connecting each point of contact have significance with regard to industry color standardization, but they do match the insulation colors of the cable I built/modified.  I’m including this illustration because the wiring diagram I used for the first cable I built didn’t work for me. serial_cable Some of the diagrams I found while searching the internet had pin-3 connected to pin-2, and pin-2 connected to pin-3 – but I couldn’t get anything to work until I tinkered with my cable to match what’s shown here.  I think this has something to do with with whether the serial devices being connected are considered Data Terminal Equipment (DTE) or Data Computing Equipment (DCE).  Also, there are some small discrepancies in the function descriptions of the table above and what is shown on page 22 the install manual.  I suppose I could really dig into this and figure it all out, but this is one of the rare occasions where I didn’t care how it worked, as long as it was working.  I know, shame on me.

To make my serial cable, I started with something I already had, however this cable included DB9 connectors on both ends.  I cut one of the ends off of this cable, and soldiered the exposed wires to a clam shell style DB25 connector I purchased on Amazon.serial_cable_DB25  Obviously it’s important to pay attention to the gender of the connector you need on each end of the cable so it mates properly with the mill and PC.  Because I modifed a DB9 cable I already had, I can’t show a picture of the inside of the DB9.  Most ready-made serial cables have molded plastic around the connectors, and cannot be opened up without hacking them to pieces.  The color of the insulation on the wires soldiered to the DB25 should match the diagram above.  Additionally, even though the cable I cut apart only had DB9 connectors on each end, which would lead you to believe there would only be (9) wires, there are actually (10).  The extra wire inside the cable is bare, and serves as a protective ground, also sometimes refered to as a sheild drain wire.  Anyway, this bare wire is soldiered to pin-1 on the DB25 connector.

This last image shows the DB9 and DB25 connectors on the finished serial cable I’m currently using on this old mill.  As you can see, the DB9 is a female connector, and the DB25 is a male connector.  I’m fairly sure the gender of the connectors shown are what most will need, but check the connectors on your mill and PC to be sure the cable you build or buy will mate up.  The total length of my serial cable is just under 6 feet, and I wish I would have made something closer to 10 feet.  It’s just a bit inconvenient at times to have my PC within 6 feet of the mill.  Oh well, I suppose I’ll get irritated enough one of these days and make a new one that is longer.  Lastly, I know that at some point a serial cable can get long enough that it causes communications issues, so don’t overdo it.


So thats it, this serial cable should get you up and going.  From here it’s just making sure the software used to send and recieve data to/from the mill is sufficient for the task and properly configured.  I’ll cover all of this on my  software connection page.

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