HP3548 / HP3457 OLED display

[Berni]

Wow nice one! It looks great!

I suppose for the HP 3458 it is a bit less attractive since it already has a VFD display(Unless you had high mileage one with a very faded VFD), but for a HP 3457 it sure does make a night and day difference. I always found non backlit LCD screens on test equipment a pain in the ass with how difficult they are to read.

I love VFD style displays on test gear. The new fangled color LCD stuff never looked good to me.

[Xyphro]

Fully agree! Vfd is really great. Actually also looked for 12 digit VFDs btw. before making this (and smaller IPS TFT panels, which unfortunately did not align well with the 12 digit pattern on its 10mm increments).
Some equipment like fluke 289 has a back Light LCD. Allthough it helps, it also reduces the contrast when it is turned on, but it still helps in some situations.

[Kleinstein]

The Hp66xx supplies have mechanical same displays, also with 12 digits. Of course supplies like E3632 and similar ones are very different.

Btw. My hp3547 shows when the input is left open in Volt Mode a very huge drifting voltage until I connect it to a source or I change the input impedance to the lower setting. Is this normal or an indication of something dying?

In high Z mode a drifting input voltage when open circuit is normal.  The 3457 is supposed to have some 220 pF plus some parasitic capacitance at the input. So a 25 pA input current would cause a dirft rate of some 0.1 V per second. If the dirft is considerably large this points to higher than normal input current. This could be from leaking parts, contamination or from a problem with the precharge circuit. The precharge part could be excluded in the non AZ mode.

[Xyphro]

 I have to disappoint in the feasibility to reuse this exact same display design as it stands for the HP66xx supplies. Mechanical it is very easy to make it fit (just a notch has to be inserted in the PCB outline). BUT (and a big one) the protocol seems different.

If I'd have all time in the world and this power supply would be extremly interesting for me to get it repaired I'd immediately jump on it to get it done, but I would here prioritize other things first. Maybe.... maybe I'd take this up in future. Decoding it should not be too tough, but time is an issue here.

[Xyphro]

2 more pictures of the supply

[Xyphro]

Actually, after a close look the protocol seems not THAT different.

Pin 9 seems to contain the data sent to the display
Pin 11 is the clock signal
HO2 is the command data stream
HO3 is the SYNC line (HIGH indicates a new command ID beeing sent, low indicates data payload)

HO0 is unknown... maybe some chip select signal?

I just did a very quick screening, but I see commands like 0x028 and 0x2F0 flying by with the above assumption.

There are a lot of unrequired pulses visible (maybe because the HSO bus of the MCU is used for other purposes too), but if you only look at the onces that are sampled on a clock edige of the Pin 11 clock signal it seems to be similar.

So maybe... I have been too negative about my previous statement?!

Attached a saleae logic trace (done with version 1.2.18)

[EHT]

The Hp66xx supplies have mechanical same displays, also with 12 digits. Of course supplies like E3632 and similar ones are very different.

Great job checking these out already. For everyone's benefit: on the 6644A, a 16-pin connector J2 on the attached diagram. I couldn't see the equivalent for the 3457A because the copy I have is pretty poor quality.

Btw. My hp3547 shows when the input is left open in Volt Mode a very huge drifting voltage until I connect it to a source or I change the input impedance to the lower setting. Is this normal or an indication of something dying?

Yes, mine shows this with no input:
* DCV about +1.4V
* ACV about 2mV

[voltsandjolts]

Well done Kai, using multiple cheap oled displays in that way is a great idea

[Hexley]

@Xyphro -
In the picture of the board that you posted in reply #4 above, there is a legend on the mounting board that says something like "Short R518 on Main PCB!"

Can you elaborate, please? I'd like to map that onto the 3478A schematic.

Or if you can just confirm the resistor number, I can chase up the 3457 schematic and see for myself; no worries.

Thanks,
hb

[Xyphro]

Hi!

This shorting is actually not required.

I use an isolator IC to avoid disturbances. That one needs to be supplied with 5V on the input side. The flatband cable to the display has a 5V line but on the HPs Digital PCB there is a series resistor to Limit the current. Depending on which Generation of multimeter you have it is either 1Kohm or 470Ohm.

I have put this remark on my pcb before I checked how low the current consumption of the isolator IC is. It works Perfect without the short circuit.

Best regards,

Kai
Ps: R540 is what I refer to.

[Hexley]

The flat and cable to the display has a 5V line but on the HPs Digital PCB there is a series resistor to Limit the current. Depending on which Generation of multimeter you have it is either 1Kohm or 470Ohm.

I have put this remark on my pcb before I checked how low the current consumption of the isolator IC is. It works Perfect without the short circuit.

It looks like that resistor is 1K in the 3478A (R508). So no need to short it, it seems. That is great. Thank you for the info.

[Xyphro]

Honestly, if I'd make another redesign I'd remove the isolator also and Power it from the 5V (then with shorted resistor). The 3.3V could be made with an LDO and the 7.5V for the Oled either using its internal chargepump (+1 transistor per oled) or a boost converter.
It would bring the complexity of the schematic down further.

[RichardM]

This looks great. Thanks.

I have an HP-3468a and was wondering if this modification would also work on it ? I will find the schematics but I had always assumed the 3468a was almost the same as the 3478a but with lower specs and HP-IL ports.

Cheers

[Xyphro]

Looking at the Service manual it might work. But should be tried out. But it is not clear from the manual what Kind of connector they use (16 pin dual row pin header or press fit dip).

[Xyphro]

Hi @All:

As promissed I have just published all design files to Github:
https://github.com/xyphro/HP3457-OLED-display

Have fun :-)

Best regards,

Kai

[Hexley]

The pdf schematic in ../hardware/eagle/ renders as though it were a mirror image, at least for me.

[Xyphro]

Haha, you are right, thanks for feedback. Will fix it :-)
Edit: DONE!

[Hexley]

Thanks, Kai.

A few more questions:

1. Can you share a part number and/or vendor information for the OLED displays that you used? I am not finding any blue on black versions, only white on black or yellow on black.

2. Your writeup mentions using a TO220 regulator and connecting AC lines to pin 1 and 2. Can you elaborate on this, please?

3. Have you had a chance to think further about what you suggested a while ago in terms of powering +5 directly from the Vcc pin of the 16-pin header (after removing the limiting resistor on the motherboard)?

Thanks for posting to github. It is great to see the details of your work.

[tfm]

This looks great. Thanks.

I have an HP-3468a and was wondering if this modification would also work on it ? I will find the schematics but I had always assumed the 3468a was almost the same as the 3478a but with lower specs and HP-IL ports.

Cheers

Hi RichardM, I am in the same situation here. And I'm willing to try it, it should work for both of them.

[Xyphro]

Hi Hexley,

1.) I ordered those displays: https://m.de.aliexpress.com/item/1005001856921229.html
Or search for: XAbl 0.87 inch oled

2.) I will share a photo tomorrow

3.) I did not investigated this further. With the galvanic Isolation and the option to power it from an AC or DC source you have the most flexibility.

Best regards,

Kai

[tfm]

Hi @All:

As promissed I have just published all design files to Github:
https://github.com/xyphro/HP3457-OLED-display

Have fun :-)

Best regards,

Kai

Thank you so much, @Xyphro! That is a great contribution!

[Xyphro]

Hi Hexley,

attached a photo that shows how I supplied the display with power.

The regulator U701 gets 12V DC on its input and outputs 5V DC. Pin 1 is the input (+12V), pin 2 is GND, the unconnected pin 3 is +5V.

I simply routed a cable from the AC connector of the new display to the regulator pin 1 and 2. As my display design has a bridge rectifier on its input, it does not matter where you connect +12V or GND to.

Best regards,

Kai
PS: The displays I bought are sold as white on black displays, but actually they look a bit blueish as you can see on the pictures.

[Hexley]

Thanks, Kai. That is very clear.

More questions:

1. Is there a special method to space the mounting board and the display board from each other when it comes time to solder the interconnecting pins?
2. Do you use anything to hold down the OLEDs to the PCB, like hot glue or super glue?
3. Do you use a jig or template to get the OLEDs centered vertically in the LCD window opening?
4. Did you try pullup resistors on the I2C lines and decide that they were not needed? [Edit: Nevermind; I see how you did the bit banging in software. Clever.]

Sorry to keep pestering you!

- Hexley

[Xyphro]

No issues, questsions are there to get asked :-)

1.) See 1st attached picture. It is simply the spacing as given by a standard pinheader - not more and not less.

2.) I used doublesided "foam tape". Not sure what the proper american term is for that. It is foam that is sticky on both sides and a bit compressable. See 2nd picture.

3.) for the orientation in Y-direction I had put 2 layers of the "foam tape" at the bottom side. I could nicely hold the OLED against it and then press it down, so that it sticks to the other foam tape. Afterwards I removed those 2 layers of foam tape (white in the picture) again. This is all I did. I did not create a template for it.

4.) I did not use Pullup resistors (a deliberate design choice). This might look hacky of course, but is perfectly OK. Yhe MCU drives I2C in push-pull mode. The SSD1306 will not do clock stretching, so the SCL line is never driven low by the display. The SDA line is driven low by the display only in the ACK phase. In that phase I do switch the MCU to open drain with pullup resistor (but actually don't read the ACK bit). So there are never 2 drivers working against each other on the SDA line. Even if this would happen, the IOs are current limited and no transistors get hurt :-) During development I had this happening a few times of course, but nothing got destroyed.

[RichardM]

Having problems posting so I apologise if this appears twice.

The 3468a appears to use a DIP press fit connector, 2 rows of 8 pins. Do these pull out or are they soldered in after pressing together ? The same connector is used on the other end on the main board.

The annunciations are also attached, the spacing between the screw holes for the LCD board are 149 mm.

How does this compare to the 3478a ?

Cheers