I know it’s 5V but what other specs should the USB cable comply with? Centre pos/neg etc.). I’m talking about the rear power (not the one in side under the plug).
It’s a tad confusing to ask about a usb cable when the USB port can’t be powered (it’s only data) - presumably you are asking about a relatively unusual USB to barrel cable and have a 5v USB psu
which provides enough current
anyway
straight from the manual
Power inlet: Center positive 3.5 × 1.35 mm barrel jack, 5 V DC, 1 A
Battery In: Center positive 5.5 × 2.1 barrel jack, 4–10 V DC
note the PSU-4 is actually rated at 6W 1.2A
stumbled on another source of Elektron info (in Beta) … via a Knowledge Base