New Raspberry Pi to 1MHz bus interface

[1024MAK]

Hi Ken,

I only just realised that you posted a schematic last year. For things like schematics, it’s better if you upload it as a zip file rather than an image file. This is because the forum software will automatically reduce the file size and hence the quality of any uploaded image files.

Mark

[KenLowe]

Noted. I've attached a higher resolution .pdf schematic to the original post.

[KenLowe]

BeebMaster want's to talk to BeebSCSI on Pi1MHz at &FC44..&FC47 (he's created a special build at this address) and a different SCSI host adaptor at &FC40..&FC43. But this doesn't work because the Pi1MHz data level shifter is enabled when either page FC or FD is active, so doesn't allow the other host adaptor exclusive access to the data bus and thereby causes bus conflicts.

To get this working I need to prevent Pi1MHz from keeping the data level shifter active when it's not actively writing to the data bus. But this would require another output from the Pi, and there doesn't appear to be any spare I/O.

I was wondering if any of the I/O could be repurposed? Having a look through the I/O, the one signal that jumped out at me was the NMI signal. Does Pi1MHz drive the NMI signal at all? If that's not available, what other options are open to us, with the limited space on the PCB.

After a quick discussion with @dp11, the ID pins were identified as possible candidates for driving the databus level shifter, so I've updated the schematic and layout to include this change. However, this will require a modification to the Pi1MHz software, so until updated software is available, I've added a jumper so the databus level shifter can be enabled either when page FC or FD is active (as per the current arrangement), or when the ID output from the Pi is driven low.

I've mentioned this on a couple of 1mhz boards, but is the audio in pin passed through in case you want to connect an external device or a DAC from the printer port?

I've also now provided this function on the board via a double height header (shared with the serial debug header).

Edit: Due to JLC shortages, I've had to swap the 74LVC2G34DRLR back out for the original NC7WZ16, and adjust the footprint accordingly (see details of the original change here). No major issue, though. Screen captures & schematic below have been updated to reflect this change (and a few other minor changes).

Constructive comments / feedback welcome...

Updated Schematic for Pi1MHz level shifter board

Updated PCB Layout for Pi1MHz level shifter board

3D Render - Front

3D Render - Rear

[BeebMaster]

Ooo! Does this mean I don't have to do this:

[KenLowe]

Perhaps. But it does need a software update to drive one of the Pi ID_SC or ID_SD pins, and I'm not too sure how easy it is to do that. Those pins are normally reserved for the Pi HAT ID EEPROM.

[Chris_68]

Hi,

I just bought the 1MHz interface, I have a Raspberry Zero 2 but wonder if there's any advantage to get a 4? I guess the RPi 5 is not supported, if it will be in the future then that might be n option, or what do you advice?

Regards
Chris

[tamemin]

Hi,

Could someone please let me know how to purchase one of the Under keyboard Pi to 1Mhz boards?

I have a Master 128 and a PI Zero spare, so would be great to have a hard disk working.

Thanks

Tam

[KenLowe]

Hi,

Could someone please let me know how to purchase one of the Under keyboard Pi to 1Mhz boards?

I have a Master 128 and a PI Zero spare, so would be great to have a hard disk working.

Thanks

Tam

Link to my For Sale thread:

viewtopic.php?t=18910

I'll drop you a PM shortly.

[KenLowe]

Hi,

I just bought the 1MHz interface, I have a Raspberry Zero 2 but wonder if there's any advantage to get a 4? I guess the RPi 5 is not supported, if it will be in the future then that might be n option, or what do you advice?

Regards
Chris

The Pi4 is not supported. Currently, only the Pi Zero / Zero W, Pi Zero 2 W & Pi 3A+ / 3B+ are supported. See here.

I wouldn't imagine the Pi5 will work either, without some software changes.

[dp11]

Pi5 needs a lot of work to make Pi1MHz or PiTubeDirect work. It's on my list. But to be honest Pi4 or Pi5 is very much an overkill for either project.

[DamoJay]

Hi all,

Received my Shifter and working great on a 3B+. Only challenge is getting the M3000 working (M5000 is working great). I came across this thread :

https://www.stardot.org.uk/forums/viewt ... 00#p395409

Which suggested to get these discs :

https://github.com/stardot/b-em/blob/ma ... us3000.ssd

And the Chariots of Fire M3000 mix off Amplex disc 26

https://www.retro-kit.co.uk/page.cfm/content/Amplinex/.

The M5000 music works great. When running Chariots of Fire, it picks up the M3000 Mix, but the sound isn't right.

I must be doing something wrong on this - any help would be appreicated - Great job on these devices !!

Thanks

Damian.

[Demon]

Hi can you get audio out of the hdmi port?
Also do you support any other audio output devices other than the headphone socket?

[KenLowe]

Hi all,

Received my Shifter and working great on a 3B+. Only challenge is getting the M3000 working (M5000 is working great). I came across this thread :

https://www.stardot.org.uk/forums/viewt ... 00#p395409

Which suggested to get these discs :

https://github.com/stardot/b-em/blob/ma ... us3000.ssd

And the Chariots of Fire M3000 mix off Amplex disc 26

https://www.retro-kit.co.uk/page.cfm/content/Amplinex/.

The M5000 music works great. When running Chariots of Fire, it picks up the M3000 Mix, but the sound isn't right.

I must be doing something wrong on this - any help would be appreicated - Great job on these devices !!

Thanks

Damian.

Sorry, I'm not sure why that shouldn't work. I'll give this a test myself when I get back home from holiday. If you're not already running the very latest firmware, which includes some improvements to the M5000 emulation, please download this from here:

https://github.com/dp111/Pi1MHz/releases

The very latest testing version can be downloaded from here:

viewtopic.php?p=424092#p424092

[KenLowe]

Hi can you get audio out of the hdmi port?
Also do you support any other audio output devices other than the headphone socket?

Unfortunately, I don't think it's possible to get audio out from the hdmi port.

In addition to getting audio out via the headphone socket, one audio channel is also routed to the beeb speaker via one of the GPIO pins on the RPi, and the audio in pin on the 1MHz bus. A copy of the schematic can be found in this post:

viewtopic.php?p=399178#p399178

[aotta]

Hi can you get audio out of the hdmi port?

Unfortunately, I don't think it's possible to get audio out from the hdmi port.

Indeed, that's what IanB's doing with RGBtoHDMI
viewtopic.php?p=424742#p424742

[dp11]

It is on my list

[maniacminer]

I wondered if the M500x emulation used the PWM outputs for audio? The PiZero can be made to output analogue stereo audio by adding the parts the Pi Foundation decided not to install. https://learn.adafruit.com/adding-basic ... ry-pi-zero it looks to be a simple RC filter. I would like to hear the M500 emulation as good as it can be, at the moment, using the Beeb speaker, it's really quiet and noisy. It should be possible to add a pin header to take the stereo audio off the 1MHz Pi level shifter and off to an audio amplifier.

[dp11]

I wondered if the M500x emulation used the PWM outputs for audio? The PiZero can be made to output analogue stereo audio by adding the parts the Pi Foundation decided not to install. https://learn.adafruit.com/adding-basic ... ry-pi-zero it looks to be a simple RC filter. I would like to hear the M500 emulation as good as it can be, at the moment, using the Beeb speaker, it's really quiet and noisy. It should be possible to add a pin header to take the stereo audio off the 1MHz Pi level shifter and off to an audio amplifier.

That's how it works. If you use a pi3A you can use the stereo audio jack.

[maniacminer]

It sounds like a new Pi1MHz board is needed with this extra feature included for the PiZero1/2. (no pun intended)