How to bit-bang 9600 baud on a Commodore 64 #Commodore #C64 #VintageComputing @pagetable

The user port of the Commodore 64 exposes a TTL-level RS-232 serial port that supports up to 1200 baud. In 1997, Daniel Dallmann came up with a very sophisticated trick that allowed sending and receiving at 9600 baud, using slightly different wiring and a dedicated driver. This “UP9600” wiring has become the de-facto standard for all modern accessories, like C64 WiFi modems. pagetable.com writes up how UP9600 works.

For cost saving reasons, the VIC-20 and its successors, the C64 and C128, did not contain a 6551 chip. Instead, Commodore included a bit-banging driver in the KERNAL that emulated the 6551 and exposed it as device #2. This emulator supports up to 2400 baud, but due to DMA from the VIC-II video chip (“badlines”), only speeds up to 1200 are stable on the C64 (and the C128 in 40 columns mode).

In 1997, Daniel Dallmann created UP9600, a solution that allowed 9600 baud on the user port. The idea is to use the hardware shift registers of the two CIA 6526 I/O controllers to do the timing-critical part of the transfer.

The highest baud rate that is possible on a C64 with the described algorithm is 9600 baud.

See all the details in the excellent article on pagetable.


Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.

Join 32,000+ makers on Adafruit’s Discord channels and be part of the community! http://adafru.it/discord

Have an amazing project to share? The Electronics Show and Tell is every Wednesday at 7pm ET! To join, head over to YouTube and check out the show’s live chat – we’ll post the link there.

Join us every Wednesday night at 8pm ET for Ask an Engineer!

Follow Adafruit on Instagram for top secret new products, behinds the scenes and more https://www.instagram.com/adafruit/

CircuitPython – The easiest way to program microcontrollers – CircuitPython.org


Maker Business — A development to watch: Intel in Ohio

Wearables — Seal the dEaL with this project tip

Electronics — Breadboarding Wire

Python for Microcontrollers — Python on Microcontrollers Newsletter: CircuitPython 7.3.0 RC0 Out and Much More! #Python #CircuitPython @micropython @ThePSF

Adafruit IoT Monthly — Detecting Water Pollution, Smart Girder, and more!

Microsoft MakeCode — MakeCode Thank You!

EYE on NPI — Maxim’s Himalaya uSLIC Step-Down Power Module #EyeOnNPI @maximintegrated @digikey

New Products – Adafruit Industries – Makers, hackers, artists, designers and engineers! — New Products 5/18/2022 Feat. Adafruit NAU7802 24-Bit ADC – STEMMA QT / Qwiic!

Get the only spam-free daily newsletter about wearables, running a "maker business", electronic tips and more! Subscribe at AdafruitDaily.com !



No Comments

No comments yet.

Sorry, the comment form is closed at this time.