RetroPrinter Support

With the ReproPrinter attached to a Raspberry Pi 3B+, the Raspberry Pi fails to boot. The power light (red) on the Raspberry Pi is on, but the activity light (green) doesn't do anything. It should flash, to show I/O activity on the MicroSD card.

The rainbow screen isn't displayed, the screen is blank.

Using a multimeter, I have discovered that the 3v GPIO pins have been pulled down to 1.84v. This is what stops the Raspberry Pi from booting, since the GPU needs a 3v power rail.

There is nothing attached to the RetroPrinter Centronics port.

If I boot the Raspberry Pi 3B+ without the RetroPrinter attached, everything works as expected. I can attach the RetroPrinter after the Raspberry Pi has booted, and everything still works correctly. Rebooting the Raspberry Pi 3B+ with the RetroPrinter attached is no problem, just the initial boot.

Certainly one other person has successfully used the Retro-Printer with the Raspberry Pi 3 Model B+

I wonder if it might be a faulty Retro-Printer module? Have you tried using the actual software image we provide just to eliminate a potential problem with the HAT itself?

I was discussing this with the hardware designer (Sascha) - when we realised that you were using your own software image.

You probably need to remove the Retro-Printer Module and disable the serial port pins, as we use those as extra input / output:
https://www.cube-controls.com/2015/11/0 ... -raspbian/

After trying several different images, Rapberry Pis and microSD cards, I'm convinced that the Pi 3B+ fails to boot because of a hardware issue.

I've always been convinced that this was a hardware issue, so I decided to build a test rig using an Adafruit T-Cobler Plus and an Adafruit Cobler Plus. I now have full access to all of the GPIO pins, to measure voltages and current flows.

With the RetroPrinter attached to the Raspberry Pi (via the test rig), I turned the power on and the 3B+ failed to boot - just as I expected. The red LED was on, with no activity on the green LED. Unfortunately, GPIO Pin 1 was no longer delivering 3.3v, it was only delivering 1.8v. I assumed that the RetroPrinter was drawing too much current from pin 1, causing a voltage drop. The next step was to measure the current flowing on pin 1.

To my surprise, the current flowing out of pin 1 was -0.028mA!!! Yes, there was a small amount of current flowing the wrong way! This would also explain why the Pi 3B+ failed to boot, since a negative current means there's a negative voltage (reducing the actual voltage on pin 1). Time to turn the power off and prevent the current flowing the wrong way.

I connected a 1N4002 diode between GPIO pin 1 on the Pi 3B+ and GPIO pin 1 on the RetroPrinter. I turned the power on and the Pi booted correctly. In fact everything worked correctly. With my BBC Micro connected to the RetroPrinter, I could print directly from the BBC Micro to my Canon Pixma MG7550 network printer.

I don't know why the RetroPrinter generates a small negative current on GPIO pin 1, but I can understand why this causes a problem with the Pi 3B+. The 3.3v power supply on the Pi 3B+ is very different to earlier power supply designs.

Now all I need to do is make a short 40-pin cable that includes a diode in the pin 1 connection.

That is interesting - I know others are successfully using the Raspberry Pi 3B+ - perhaps you could confirm the version number of the Raspberry Pi and we could compare it with ones which are known to work!

There is also an issue with using one of the existing Retro-Printer SD images on the Pi 3B+ - we hope to have a new image to download soon

We think that the power on pin 1 of the GPIO may well be due to the factory error when soldering the centronics port connector, but will add a diode to pin 1 for future revisions of the PCB.

Interestingly a user with PI 3 Model B+ V.1.2 2015 did not experience this issue.

Sounds like someone is getting a little confused with Raspberry Pi model names and versions. The Raspberry Pi 3B+ was released on March 14th 2018 (Pi day). The distinctive features of the 3B+ are the metal heatsinks covering the CPU and the Wifi chip. The version number for the 3B+ is V1.3. Anything with an earlier date, or a lower version number, must be an earlier Raspberry Pi model B. As always, there's a Wiki full of information that helps with model names - https://en.wikipedia.org/wiki/Raspberry_Pi

Yes, I thought that too. I thought the 3B+ was a bit more modern and the 2018 release date would fit in with the datestamp of 2017 on the PCB.

Did you manage to attach the diode? Any pictures?

So here are the pictures of how I attached a small diode and a piece of wire between GPIO pin 1 on the bottom of the Raspberry Pi 3B+ and GPIO pin 1 on the top of the RetroPrinter. Note that I have cut into the connector on the bottom of the RetroPrinter, to break the pin 1 connection. Everything fits neatly inside the RetroPrinter case.