Commodore 16, 116, & Plus/4 Technical Details and Trivia


  • a cartridge/expansion port that allows full MOS8501 bus access - 50-pin edge connector. On the C116, this was used to connect the VIC-1551 disk drive.
  • two joystick/paddle ports, 8-pin mini-DIN sockets.
  • a composite video and audio output socket - 8-pin round DIN.
  • a UHF TV output socket - standard RCA connector
  • a cassette interface running at 300 baud. Compatible with earlier PET "Datasette" units - 8-pin mini-DIN. On the C116, this was a 7-pin mini-DIN.
  • a "user port" for TTL-level RS232 signals. Can be used for modems, parallel printers and more - 24-pin edge connector
  • a reset button
  • a serial bus for CBM disk drives and parallel printers. Serial version of IEEE-488 - 6-pin DIN
  • a power supply socket - on the Plus/4 it requires 5V DC and 9V DC from external PSU - 4-pin square-DIN. On the C16 the single-pin round DIN socket requires 9V DC @ 1A.

Technical Facts

  • all 264-series computers (C16, C116, and Plus/4) used the same CPU - a MOS Technology 7501 or 8501, both of which were modified versions of the MOS 6502, found in the Acorn BBC computer. The MOS 7501 was manufactured using HMOS-1 technology, and MOS 8501 using the newer high speed HMOS-2 process. Both ICs are opcode-compatible with the more common MOS 6510 used in the Commodore 64). MOS 8501 CPUs are quite difficult to source these days. The very last batch of 8501s apparently used the NMOS manufacturing process, which made them run cooler than HMOS chips.
  • the TED (Text Editing Device) chip caused the C16 and Plus/4 computers to run slower in some modes, because it did not have a seperate colour [video] RAM to store its data. Instead it used the computer's main memory, which is accessed by both the CPU and TED. If you tell TED to disable the screen output (border color displayed on the whole screen), the CPU will run on 1.7 MHz all the time.
  • TED also handled the sound, omitting the brilliant SID chip as found in the C64. TED, however, was not as capable sound-wise, offering just 2 tone generators, although one of these could also produce distortion effects.
  • The Plus/4 could run both the standard serial floppy drives (CBM 1540/1541) and it's own dedicated drive, the CBM 1550, which was much faster (400%!) due to its use of the parallel interface of the cartridge port. Commodore also produced the CBM 1542 which was just a repackaged 1541 in a grey case for people who didn't want to spend the extra money for the 1551. Since the 1551 disk drive is a parallel device, it actually connects to the Plus/4 and C16 via the expansion port, whereas the other Commodore disk drives are serial devices so they use the serial port.


Hover your mouse over this Plus/4 circuit board for a description of the components

Component details reproduced with friendly permission from Sothius' Home, (shame you closed your site, Sothius, it was great! Ed)


Hover your mouse over this Commodore 16 circuit board for a description of the components

Component details reproduced with friendly permission from Sothius' Home,



  • The Plus/4 was originally planned to be a professional business computer (as they already had their games machine in the C64). For this purpose, the machine had 'option ROM' sockets (similar to the Acorn BBC and Commodore's own PET-series). The Plus/4 came with four integrated programs on two ROMs - Tri-Micro's '3-plus-1' application suite, consisting of a word processor, spreadsheet, graphics tool and file manager.
  • The Plus/4 was called the C264 during development, and was named 'Plus/4' due to the four built-in applications.
  • The standard floppy drive for the C16 and Plus/4, the CBM 1551, makes use of the machine's cartridge port. This port has special disk I/O features for parallel data transfers, making it much faster than the 1540/1541 serial interface.
  • You can bring up the built-in machine-language monitor tool when in BASIC, by typing in 'MONITOR' and hitting Enter.
  • Earlier Plus/4s came with a round DIN socket for power, which allowed the use of the C64's power supply. This was later replaced with a 'square-DIN', for use with the 310200-09 power supply.
  • Commodore never produced any RAM expansion solution to the C16, although several third-party companies produced them.
  • Both the C16 and Plus/4 came with an enhanced version of BASIC - Commodore BASIC v3.5. This was a big improvement on the older version from the C64, and included specific commands for disk I/O and TED including control commands for graphics, sound and bank-switching. The later Commodore 128's BASIC v7.0 took this version as its basis although omitted the TED-specific stuff.
  • You could actually connect two disk drives to the C16 and Plus/4 computers. The second drive cable attached to the back of the first cable.
  • To hard-reset a C16 or Plus/4, you press and hold RUN/STOP and CTRL, and press the reset button.
  • Eary C16 motherboards (revision "A") couldn't be expanded out-of-the-box to use extra memory via the cartridge port. These motherboards required new diodes to be added before the memory would work, some didn't. In fact, some third-party memory boards came with diodes in the box for exactly this purpose! It is thought that Revision B motherboards supported memory boards without this need.


Troubleshooting a Dead C16, Plus/4 or C116

There are 3 main reasons for a dead 264 system - CPU (7501 or 8501), TED (7360 or 8360) and PLA (251641-02).
All three are prone to overheating.
Your first test should be to identify whether you're getting anything from the machine or not. Check the fuse in the plug first of all, then the fuse inside the computer.

Once you know you're sending power to the computer, check if you get any faint scan lines on your TV when you switch it on. If so, it's likely the TED chip is dead. Try pulling and reseating the TED in its socket. Carefully clean up the pins on the chip and the receiving connectors in the socket if they're tarnished.

If there is really no activity from the computer (no scan lines at all on the TV) or a completely black screen, and you've checked the TED is good, it's likely the CPU is faulty. If the unit appears to power on (power LED lights up) but no image, this too is a sign of a bad CPU.

This page was last updated on 8th April 2015.