With serial communications there are two standard types of cables. Those are null modem and straight-thru cables.
Straight-thru cables used to be very common and were typically used for devices where the communications all went one direction only at any given time. Think downloading or uploading to and from a device.
Null modem cables were commonly used when you needed communications to be possible in both directions at the same time. Imagine you want to go online to a PLC and view what is happening and change a timer or setpoint value.
You can add a null modem adapter to a straight-thru type cable and you then have a null modem cable. This used to be really common so you didn't need to carry two types of cables. Less commonly, you could add a null modem adapter to a null modem cable and you then have a straight-thru cable. I can't say that I have ever actually tried that though.
The 1747-CP3 is a null modem cable Rockwell sells (assuming they still sell it). But any null modem cable will do the same thing. So don't feel obligated to buy that cable from Rockwell. The one thing I do like about that cable is that it has a right-angle connector so you can close the front door on the SLC controller. Unfortunately when Rockwell came out with the newer Logix style controllers this cable was too big to allow the door to close. So Rockwell came out with the 1756-CP3 cable which was shallower and permitted the door on the ControlLogix controllers to close with the cable attached. Same cable as the 1747-CP3, just a smaller connector body.
So if you do want to buy from Rockwell, look and see if thy have a price difference between the 1747 and 1756-CP3 cables. They both work exactly the same.
The big issue today is the lack of a serial port on our computers. That is where he USB adapters come in. Of course Rockwell is happy to sell you theirs. And I have one and have used it without any issues for years. But I worked for them at the time and didn't have to pay for it. If I am spending my own money or my company's money, I am going with the link previously posted above which I will repeat here
https://www.plccable.com/allen-bradley-slc-500-usb-1747-cp3-1756-cp3-aftermarket-9300-usbcbl-cp3/
Forum member geniusintraining won't promote himself here, but he is the man, the myth, the legend behind PLC Cables. Forum members here have been raving about his products for years.
OG