The cable is manufactured by The Retro Channel (TRC) based on the schematics compiled by Levente Hársfalvi (TLC), and sold via [Tindie], priced $35.00 (+p&p +VAT).
This is the description as reported from the official product page:
"Yep, why Commodore used mini-DIN rather than the de-facto standard DB9 (ackchyually DE9) is anyone's guess. They also incorporated a select line which isn't present on standard joysticks. A solution was needed to solve both, one that would fit in the tight space between the ports on all machines.
That's where this thing comes in. It plugs straight into your Commodore and provides standard joystick ports. And unlike cheap passthrough adaptors, this correctly handles the select line, preventing any possible damage to the TED. Better yet you can leave it in and forget about it, no need to go searching around for that damn C264 joystick adaptor.
This is based on the document by Levente Hársfalvi who descibes in detail how the joystick ports work on these machines and how to correctly handle their logic.
Please note that due to how the joystick interface works on the 264 series, autofire joysticks are not guaranteed to work correctly. Also Genesis/Mega Drive controllers should not be used with this adaptor (and most other systems for that matter)."
