Building your PicoGUS
This is a guide for building your own PicoGUS 1.2 board. If you want to buy a pre-made PicoGUS 2.0 board, they are available from these online stores:
- Joe's Computer Museum Shop - in US 🇺🇸
- Serdashop - in EU 🇪🇺
- Flamelily IT - in UK 🇬🇧
But if you want to make your own 1.2 board? Continue on below!
Upload a zip of the hw/gerbers directory to your favorite PCB fabricator. The design rules are to JLCPCB's 2-layer PCB service standards, and should also be handled by almost any good-quality PCB house.
Specs I've built my prototypes with at JLCPCB:
- Soldermask: red (the original Gravis Ultrasound was red, after all!)
- Surface finish: HASL w/ lead. I've heard both ways about whether HASL or ENIG card edge connectors last longer. ENIG won't corrode, but is an extremely thin surface. HASL is thicker and can possibly take more wear. Hard gold is better than both, but is not an option at JLCPCB, and is expensive at fabricators that do offer it like PCBWay.
JLCPCB also offers very reasonably priced assembly services, with parts sourced from LCSC. Once an absolute final board design is ready, I will create appropriate BOM and assembly files ready for upload to JLCPCB.
See the hw/bom.csv file which has part numbers for both Digi-Key and Mouser. For many of the parts, the manufacturer doesn't matter and can be substituted, but for the 74CB3T parts, they are only made by TI. In a pinch, U2 can be substituted with an 74LVC244 part. It theoretically has different timing than the mux chips but I have tested it with no performance difference. The BOM also specifies a blank card bracket that must be drilled. See the mounting bracket section below for a better 3D printable option.
For your convenience, here are projects ready to be added to cart and checked out:
One part not available in from Digi-Key or Mouser is the GY-PCM5102 DAC board. Search Amazon, eBay, or Aliexpress for this part number, or use the following affiliate links which should have good availability from reputable sellers. I have used both of the boards from these links successfully. If you order from the links I will get a cut of few cents.
The PicoGUS uses mostly surface-mount components. The TSSOP components (U2, U3, U4) are quite fine pitched but with care can be hand-soldered. Just use plenty of good quality flux and a small amount of solder, using a drag soldering technique. Be careful of solder bridges, which I've experienced hiding out under the TSSOP legs. Inspecting your solder joints with a strong magnifier or microscope is a very good idea.
An interactive BOM to help you locate components during assembly is available at https://polpo.org/picogus/.
I recommend assembling the board in this order:
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SOIC components (U5, U6, U8). This should prepare you a bit mentally and physically for the TSSOPs that come next...
Watch out for component orientation – the footprints I used have relatively subtle indication for where pin 1 is. Look for the longer line.
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TSSOP components (U2, U3, U4). Remember to look for bridges!
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SOT23-5 components (U7, U9)
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Passive 0805 components (C3-C10, R1-R4, L1, L2)
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The power protection diode (D1)
This is another component with an ambiguous footprint. The cathode of the diode, marked with a line or set of lines on the package, goes towards the top edge of the board.
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The Raspberry Pi Pico (U1)
There are several ways to mount the Pico. The easiest is to surface mount the Pico by directly soldering the castellated edges (remember to use flux). Pin headers are also an option. This is what I use to allow for easy attachment of logic probes. If you wish to reuse your Pico for other projects or are using a Pico H that already has headers on it, pin sockets are also an option.
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Headers for the IRQ/DMA jumpers, UART TX, and I2S_DAC (J1, J6, I2S_DAC)
For the I2S_DAC component, headers for the 6 pins along the bottom are required, along with 4 for reinforcement on the top left side. You can see the headers in the fully assembled board here:
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Solder the I2S_DAC module onto its headers
Before soldering it onto its headers, check that the GY-PCM5102 DAC module has its solder jumpers on its bottom set correctly. Most come with the proper setting but some testers have gotten DAC modules with no jumpers set. Refer to this image for the proper settings (1 = L, 2 = L, 3 = H, 4 = L):
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Finally, the tallest components: the electrolytic capacitors (C1, C2) and the MIDI out jack (J5).
Note polarity on the capacitors. The side with the negative pin is marked with white.
I originally specified a Keystone Electronics 9202-1 ISA card bracket originally made for modems in the BOM, but they happened to go out of stock at Mouser the same day I released this guide, and more probably won't be made. The BOM now includes a variant of the bracket with no holes that must be drilled. Fortunately, Todd Gill of Retro Frog has graciously designed and made available in the public domain a 3D printable bracket made just for the PicoGUS. Thank you Todd!
Note that on the current hardware revision, the RESET line on the ISA connector will be floating if the card is out of a machine, which can prevent the Pico from running and being put in programming mode. You can either program the Pico before mounting it on the PicoGUS board, or after assembly by temporarily connecting the RESET line on the PicoGUS to ground during programming. Conveniently, RESET and ground are adjacent to each other on the ISA bus breakout footprint, circled below. They can be connected with a small wire or resistor, or you can add a jumper header across these pins that can be connected to make programming easier.
Download the latest software release at https://github.com/polpo/picogus/releases. The UF2 file can be programmed by holding down the BOOTSEL button on the Pico while connecting it to a computer with a micro USB cable. Copy the picogus.uf2 file to the drive that appears on your computer, which will self-eject once the file is copied.
Once the Pico has been originally programmed, the firmware can be upgraded from DOS with pgusinit.exe's /flash option.
Now that you've made and programmed your PicoGUS, now is the time to use it! Refer to the Configuring and using your PicoGUS page for instructions.