The SB5 does not have a program track mode, or a program track output. You can however do programming on the main. The SB5 is designed as the second purchase / upgrade after the initial Powercab purchase.
The normal / recommended setup is as follows:
Part 1. - The SB5 and its power supply are connected to your main layout full time, and maybe a UTP panel or two connected via the cab bus jack. You use the Powercab or any other NCE cab connected to a panel. Trains with remain running when the Powercab is unplugged since the SB5 is now powering the layout. The Powercab is now being used as a regular cab / dumb terminal. You use the coiled cable with the SB5 since the Powercab is being used as just a cab.
Part 2. – On your workbench you use the Powercab as a programming and testing workstation. This is where you use the original configuration, powercab panel (PCP), flat cable and power supply. The Powercab gets plugged in on the left side with the flat cable. LED below the jacks.
The Powercab will be used in both locations but in different ways, all you have to do is unplug it from one location and plug it into the other. So now you have two completely separate areas that do different things just by unplugging / moving the Powercab around. You can also get ANY other NCE cab to run trains on the big layout, or even go wireless.
A programming track mode and a program track are essential for the ability to READBACK a CV. Read back is not available on the main.
Some decoders have limitations on what items can be changed in POM mode.
The best solution for you is to setup a dedicated test track / workbench track with the Powercab and the Powercab panel (pcp). Add the USB interface to this for programming via a PC / JMRI. Add the SB5 smart booster/command station to run the layout. The Powercab will function as a cab when used on the main layout and as a command station / programmer on the test track. Once you have the SB5 in place, you can also upgrade to wireless cabs later if you want.