Respectfully Modifying the Amiga 500

Modifying the Amiga 500 to speed up access to RAM in a memory expansion pack is a well documented procedure, with guides on the process written in the early 1990’s when the hardware was only a few years old. But as they were written for contemporary hardware, they make no concessions for how one should be treating a vintage computer that’s now over 30 years old. In 1993, cutting traces on the Amiga 500 motherboard was just a last ditch effort to eek a few more months of service life out of an outdated desktop computer. But in 2018, it’s kind of like when that old lady tried to “restore” a fresco of Jesus in Spain; it might be done with the best of intentions, but you still screwed the thing up good and proper.

Such things don’t fly over at [Inkoo Vintage Computing]. There you can find a guide that details the impressive lengths one can go to if they want to perform the classic modification without any irreversible changes to the motherboard. To avoid the cut traces and soldered bodge wires, this version of the modification makes use of a novel adapter that breaks out the necessary connections on the 8372A chip.

The adapter is simply a homemade PCB with both male and female plastic leaded chip carrier (PLCC) connectors. The few pins on the chip that needed rerouting are exposed as solder pads on the adapter for easy wiring. There are even a couple jumpers on the adapter to turn the modifications on and off.

Not surprisingly, the trickiest part of building this adapter was sourcing the antiquated PLCC connectors. Assuming you can even find them, you are then left with the challenging task of soldering them together. Judging by the pictures on the [Inkoo Vintage Computing] page, it’s no walk in the park.

Another similar arrangement is used in the expansion bay of the Amiga, where a pin is virtually “cut” in the connector. A tiny PCB is soldered to a 3×2 header to reroute the signals, and another jumper is used to enable and disable the pin. Luckily, the long pins on the Amiga memory expansion are forgiving enough that the little board can fit in between them without breaking electrical contact.

We’re no stranger to the Amiga 500 around these parts. We’ve covered how to get the 1987-vintage machine online in the 21st century, as well as employing a Raspberry Pi to emulate the original floppy drive. You can even make your own faux-Amiga with a 3D printed case, if you suffer from a sort of existential dread when working on a computer that’s older than you are.

I2C Bus Splitting with a more Professional Touch

Last week, I covered some of the bitter details of an interesting hack that lets us split up the I²C clock line into multiple outputs with a demultiplexer, effectively giving us “Chip Selects” for devices with the same address.

This week, I figured it’d be best to layout a slightly more practical method for solving the same problem of talking to I²C devices that each have the same address.

I actually had a great collection of comments mention the same family of chips I’m using to tackle this issue, and I’m glad that we’re jumping off the same lead as we explore the design space.

Recalling the Work of Our Predecessors

Before figuring out a clever way of hacking together our own solution, it’s best to see if someone before us has already gone through all of the trouble to solve that problem. In this case–we’re in luck–so much that the exact bus-splitting behavior we want is embedded into a discrete IC, known as the PCA9547.

chip_reverence

It’s worth remembering that our predecessors have labored tirelessly to create such a commodity piece of silicon.

The PCA9547 (PDF) is an octal, I²C bus multiplexer, and I daresay, it’s probably the most practical solution for this scenario. Not only does the chip provide 8 separate buses, up to seven more additional PCA9547s can be connected to enable communication with up to 64 identical devices! What’s more, the PCA9547 comes with the additional benefit of being compatible with both 3.3V and 5V logic-level devices on separate buses. Finally, as opposed to last week’s “hack,” each bus is bidirectional, which means the PCA9547 is fully compliant with the I²C spec.

Selecting one of the eight I²C buses is done via a transfer on the I²C bus itself. It’s worth mentioning that this method does introduce a small amount of latency compared to the previous clock-splitter solution from last week. Nevertheless, if you’re planning to read multiple devices sequentially from a single bus anyway, then getting as close-as-possible to a simultaneous read/write from each device isn’t likely a constraint on your system.

 

With a breakout board to expose the pads, I mocked up a quick-n-dirty Arduino Library to get the conversation started and duplicated last week’s demo.

Happily enough, with a single function to change the bus address, the PCA9547 is pretty much a drop-in solution that “just works.” It’s definitely reassuring that we can stand on the shoulders of our chip designers to get the job done quickly. (They’ve also likely done quite a bit more testing to ensure their device performs as promised.) Just like last week, feel free to check out the demo source code up on Github.

Until next time–cheers!

I/O extender board and case for a cheap WiFi router

This 3d printed case houses the already small [TP-Link TL-WR703N] but also makes room for a custom expansion board. The expansion board is designed to make the device more hacker friendly, and who doesn’t need a nice case to hold it?

Since the router board already has a USB port (intended for use with USB 3G modems) the add-on acts as a USB hub. The stock USB connector is replaced by a pin header which mates with a DIL socket on the underside of the expansion board. Through the use of an FTDI chip the expander offers three USB ports and a 2×10 pin header to break out the GPIO pins from the router’s processor. Only two USB ports are visible in the image above. That’s because the third is recessed, and an opening has not been added to the enclosure. This struck us as odd until we read that the port is meant to be used with a low-profile thumb drive, essentially adding internal storage for the device.

[Thanks buZz]