Why were some DC adapters center ground, shield positive?

[Chuck(G)]

That really is funny--some MCUs have two USB ports, both OTG-capable.

 

[krebizfan]

The standard for OTG from 2012 says

There are two categories of Targeted Hosts:
1. Embedded Hosts: An Embedded Host (EH) product provides Targeted Host functionality over one
or more Standard-A or Micro-AB receptacles. Embedded Host products may also offer USB
peripheral capability, delivered separately via one or more Type-B receptacles.
2. On-The-Go: An OTG product is a portable device that uses a single Micro-AB receptacle (and no
other USB receptacles) to operate at times as a USB Targeted Host and at times as a USB
peripheral. OTG devices shall always operate as a standard peripheral when connected to a
standard USB host.

If there is more than one port on the device, none of the ports are OTG ports. Except of course, no one bothers with the standard because the USB standards involve an idealized model.

 

[Chuck(G)]

"Except of course, no one bothers with the standard because the USB standards involve an idealized model."

That holds particularly true for Far East origin devices. Few manufacturers are members of USB-IF; in fact, VIDs and PIDs are either made up or blatantly copied. How many flash drives from China have VIDs that claim that they're "Kingston" for example. Fry's Electronics had the temerity to sell flash drives with the VID of 0001.

 

[Slob]

Center negative is common in guitar pedals for switching as OP described. This seems to carry over into other gear like Roland MIDI modules.

I have a several pieces of 1960's portable audio equipment (tape recorders, transistor radios) with positive ground adapters. I think that this fell out of favor in the 1970's.

At one time, most cars had 6V positive ground.