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SwitchTOS Assembly Guide

Important: The PCBs are prepared for different assembly variants, not all component positions require components. If TOS switching by reset button is planned, no pull-up resistors and no jumpers are populated. First, familiarize yourself with the schematic, bill of materials, and PCB.

Practical help for soldering SMD components can be found in all kinds of tutorials on the Internet, e.g. www.mikrocontroller.net

This assembly guide only covers the hardware side. Which TOS versions the user gets from where and how he appends them and gets them into the two Flash is up to him. Even an ATARI ST could program 5V flash on the cartridge port if you allow write access to the cartridge port, e.g. by a small insulating plate in the GLUE socket, which suppresses a bus error.

Most of the components can be ordered in our webshop (note the minimum order value!)

  1. Solder the two PLCC32 SMD sockets on the BOTTOM side. When soldering by hand, remove the base plate from the socket with a fine side cutter or similar beforehand in order to reach all pins with the tip of the soldering iron. Do not hold the soldering iron too flat so as not to disfigure the edges of the socket.

  2. Solder the remaining SMD components.

  3. Carefully pry 6 ROMs out of the sockets in the ATARI and store them safely. If you want to make your existing TOS one of the 4 selectable TOS versions and don't want to download the binaries from the Internet, these 6 ROMs will still serve as a master for a flash block.

  4. Solder two angled pins opposite each other into one of the DIL28 footprints and temporarily insert the board into the de-energized ATARI. Then insert the ATARI power supply and check that the power supply does not collide with the SwitchTOS board. Only when the right pins have been found for mounting according to the next point, continue.

  5. Try to use auxiliary devices (female headers, IC sockets, breadboards) to keep as many pins as possible in a defined shape so that you don't have to realign each pin. This is much easier with pins that are plugged through than with pins that have to be soldered completely on the BOTTOM side. If the pins protrude beyond the solder tabs, shorten before soldering. Pull out angled pins from suitable pin headers with pliers and solder them onto the wide flags on the DIL28 pads.

    1. Insert longer pins through the holes and solder the long arm on the top side

    2. Do not pass shorter pins through the holes, but solder them completely on the bottom side. The bend of the pin must be below the hole so that the grid is maintained.

    3. Pin headers instead of individual pins would be easiest to solder. We only recommend this option if you have angled pin headers with a 2.54mm pitch and 0.5mm pins. The standard headers have pins with 0.65-0.7mm pins. Although they fit through the holes in the board, they are actually too big for the sockets in the ATARI. The board would only be pressed against the socket from above but would hardly snap into the socket.
  6. Solder only as many pins as necessary.The data pins and OE pins must be connected in both DIL28 footprints, the address pins only in one footprint (see schematics).
  7. Checking the pins to the same length in the z-direction is a critical step. While the low-cost sockets in the ATARI are more forgiving of an x-y offset, an offset in the z-direction ends with an interruption. The longer pin already touches down in the base and the shorter pin has no contact yet. The result can be a floating address or data line that has to be laboriously tracked down.
  8. Make two jumpers on the SwitchTOS board (black in the picture below), on the TOP or BOTTOM side.
  9. Insert the programmed two flash ICs into the sockets. Because of Big Endian in the 68k world, think carefully about which IC goes where. The markings H and L on the PCB refer to the data bus, so H is the higher data lines, L is the lower ones. If, for example, you use ROMSPLIT to separate high-byte and low-byte, then the result refers to the addresses, the high-byte is stored in the low-address. Consequently, the L-chip from ROMSPLIT must be inserted into the H-socket and the H-chip into the L-Socket. If you don't observe it, the ATARI simply won't boot, but will eventually count through the entire address space. If you want to imagine it: the first byte is on the side of the power button, the second on the side of the keyboard. At this point, it is recommended to thoroughly check all signals from the IC pins to the angled pins.
  10. Two address lines lead to the resistor network on the CPU. Two soldering points on the ATARI are unavoidable here.
  11. Two CE lines lead to the other sockets.It doesn't matter whether the upper or lower socket is contacted.                              
  12. If you want TOS switching with the RESET button, then make a pluggable connection to the dual timer 556. A soldering point on the ATARI is unavoidable here.
  13. The 4.7k resistor visible at the bottom of the board does not appear in the schematic and is optional. Address lines A9-A16 are terminated different in ATARI as A17-A24. If you want to see the same signal levels at A16 and A17, you should add the resistor to A17.
  14. Before inserting the power supply, glue an insulating foil to the metal surface above our SwitchTOS. If SwitchTOS should ever jump out of the DIL28 sockets during vibrations, short circuits are avoided.