Rewiring MAX232 power

[Mitchell Wolrich]

Oscar, the Raspberry Pi 5 has 2 serial ports now, the standard one it's always had, and a new "Debug" serial port on a tiny 3-pin header located between HDMI0 and the HDMI1 connectors.

I located some of the tiny connectors at Amazon, with with 3 wires (as you would expect TX, GND and RX).

However, their is of course an issue.  This port is strictly 3.3V tolerant, so I can't use +5 to connect it to the MAX232 RS232 level shifter..  I tried using a test MAX3232 on one of the DB9 boards you get from China, the PI goes into a continous reboot cycle..  when I power it from the 5V power pin.  If I switch it, so the MAX3232 is powered from the +3.3V pin on the 40 pin connector, it works as expected, the PPI boots normally.

So I just received my kit, and used an ohmeter to see how you connected the "MAX232" (as you have it labeled), it's connected to the +5 power.

I need to cut this trace, and repower the MAX232 from the 3.3V pin, I did some tracing on the board, and it looks like the +5V power comes from the diode above the 74HC238, over to the C5 decoipling capacitor, then to the power pin on the MAX232 (Pin 16).

So I plan to cut the trace right at C5 + connection, and connect the 3.3V power to C5, which will power the MAX232, and then the diagnotic serial, and the standard serial will work (it really should be powered by 3.3V)

Can you verify I am correct here?   It's also a suggestion for the next board run, to wire it to the +3.3V, as most will use the RPPI 5 with the PiDP10, and then you can get both serial ports working on it.   I also think the RS232 DB9's should be wired up as well, but that's another issue, running 3 wires isn't too bad...

Thanks,

Mitch

[The Oracle]

Mitch,

Quick question, in the kit building instructions I see DB9 connectors but if you look at photos of the system I see a DB25 on the back. Which one is used, the DB25 is what I'm setup for in my lab? 

Thanks,

Lynn

[Mitchell Wolrich]

You can wire it up either way, both are their..   I prefer DB9's, as I have mostly PC's..  there are good instructions on wiring the MAX232 up here..

https://groups.google.com/g/pidp-10/c/RLPr3TK7KVM/m/U5pDQkhvAgAJ

just wire it up for the DB25 using TX & RX on that (Pins 2 & 3), ground of course is on Pin 7 on the DB25.

I think Oscar could have wired up both, and then just solder iyour prefered connectors, but he had limited space to route traces, so running a few wires won't matter for the few people who care to wire up the (2) RS232 ports on the Raspberry Pi 5

Mitch

[Mitchell Wolrich]

Oscar, I have confirmed the MAX3232 doesn't operate correctly at 5V, when using 115K baud.. I'm getting a lot of garbage characters on both of my serial terminals, these worked when using the Chinese RS232 DB9 with the MAX3232 on them, but I ran them at 3.3V, that's the only difference..  I need to cut the +5V line to the MAXIUM chip and run a jumper to feed it with 3.3V, I put my suggestion in the first post, thoughts?

This is the Diagnostic serial port between the 2 HDMI connectors and the standard one you routed over near the I2C external interface.

Also as far as running the (2) 5V fans from the connectorion on the PCB, there isn't even enough room between the display board and the rear panel to use right angle connectors, I ended up solder wiring to it, with matchin connectors that the Fans use, and I may have to extend them (which presents me an the option to put a small switch on each fan, if I want to shut them off)

Also the gentlement who stated the rear dress cover doesn't fit, is correct, the Raspberry Pi heatsink/fan assembly has 2 nylon pins that protrude, so I'm thinking of drilling a hole in the dress voer for eahc one, and just let them protrude.

I'l post some pictures of the RS232 to DB9 wiring as well when I have more time, about to solder switches :-)

Mitch

[Mitchell Wolrich]

Diagnostic serial port wired up

point to point wiring for both serial ports

the black sharpie on the LEDs was marking them after the testing, one was dead, the other glowed orange, different than all other LEDs, both were changed out

Mitch

P.S. 115K baud still getting garbage, I believd MAX3232 needs to be powered with +3.3V for compatability with RPI interface levels

[DR]

This wiring is beginning to make the kits look like what the 'real'
computers looked like inside decades ago!

[Mitchell Wolrich]

Tonight I cut the +5V trace that feeds the decoupling capacitor and the VCC pin into the MAX3232, the results are mixed.

The RPI5 Diagnostic serial port now works perfectly, I get diagnostic output on it, on bootup and I can login to it from a serial terminal at 115K baud, no issue with transmision or garbage characters.

On the RPI5 built in serial port on the 40 pin connector, it "displays" correctly on serial termiinal, however, input from the keyboard now isn't working..  the dispay looks good though, no garabage characters, as I had when the MAX 3232 was runing on +5V

So I am befudled... I thought the inputs and outputs from the RPI5 where 3.3V compatabile, so why am i having this "mute'" issue on the built in serial port?  I know it works because i was getting junk / garabge when I typed on it before the change to +3V, so the wiring is correct.

BTW, the Diagnostic serial port is known as "ttyAMA10", while the standard RPI serial port is "ttyAMA0"

Anyone have any ideas?

Thanks,

Mitch

[Mitchell Wolrich]

results of more testing.

I wired the MAX3232 back to +5 and disconnected the 3 pin plug to AMA10 (Diagnostic port between HDM1 and HDM2), as this port is ONLY 3.3V tolerant, the PI does constant reboot loops, and does not want +5V logic levels for that diagnostic port).

The results are that AMA0 (standard serial on the 40 pin connectors), now works, at the +5V level, I do get an occasiional scrambled character on input only at 115K baud, but at 3.3V, the input didn't even work, at all.

So the Diagnostic port runs on 3.3V levels, while the standard serial port on the 40 pin runs at 5V logic levels

My solution will be using one of these 3.3V to 5V voltage shifters 

https://www.amazon.com/gp/product/B07LG646VS/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I will run the MAX3232 at 3.3V and shift AMA0 to +5V logic levels, and see ow that goes.

Mitch