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HP/Agilent 53131A/53132A/53181A Conversion to LED Displays

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TomT:
Hi all,

With the popularity of these counters, I feel that might be prudent to look into designing an LED replacement. The lack of new displays, questionable life expectancy of used ones,  and overall short life of new displays compared to LED's has been of concern since I purchased my counter on eBay.  Fortunately, my display is in good condition, but don't want to wait until the last minute.

The original interface is relatively simple, as all of the decoding is done in the processor on the main board, and clocked into a 32 bit shift register/latch, with high voltage drivers (SN75518FN [obsolete]) on the front panel PCB for the VFD.  All segments and annunciators, as well as digit enable (grid drive) are included in this string.  Since the VFD requires a positive voltage to the grids to enable a character, and positive voltage for each segment/annunciator (plate/anode), all logic levels shifted into the shift register are positive true.

Due to space limitations between the tops of existing parts on the PCB, and the front panel, I am looking at 0.4" 14 segment SMD LED displays, but they only contain decimals, and the segment width is 1mm.  The addition of an upper dot and comma would have to be made.  I only have couple ideas here.  One is to contact some display manufacturers to make single/multiple character displays with the punctuation built in.  Obviously, that would take a rather large order, but if enough people got involved, this could reduce the cost of the displays.  The other option is to make a black polycarbonate/acrylic with translucent pieces for a dot and a full comma.  Logic would have to be used to turn off the decimal point when the comma is in use.  I am not a mechanical designer, so help here would be greatly appreciated.

As for annunciations at the bottom and right side of the display, I expect the best method would be rear or side lit engraved flat black painted polycarbonate/acrylic.

The right most digit is set up in a somewhat unusual configuration.  I haven't decoded exactly what-is-what, but there are more segments used for annunciators than can be accounted for in the outputs of the shift register.  Annunciators M, Hz, ยต, S, Gate, and ExtRef eat into the character segments.  Three use the punctuation, and two are specifically for annunciators, so one of the character segments is required for one of the annunciators (i.e. the lower part of the comma is used for the Gate annunciator).

I have two interface design ideas.  One uses 4x 74HCT595's or HC595's, the other uses an MM5450/51 (34/35 bit LED display driver [shift register/latch/driver [w/brightness control]).  Due to current handling limitations of the HC595/HCT595, external driver transistors would be required, as the absolute max current through VCC or GND is only +/-70ma, and the MM5450/51outputs only have a 15ma pull-down capability.  The TLC6C598 looks like a very good 8 bit x4 option, but none are expected until 2023 at Mouser, not indicated at Digikey, and Newark has SOIC-16 stock expected the week of 12/6/2021.   Other suggestions for IC's would be appreciated.

I am about to order displays from Digikey, and will breadboard it to operate in parallel with the current display in my counter.  I am using RS-485 drivers and receivers, so the counter can remain in its normal position while work is done on the circuit.

I have also looked into upgrading the 34401A 6 1/2 digit DMM's, but that circuit may be a nightmare waiting to happen.

Any suggestions, or current plans for a project like this would be welcome.

TheSteve:
I'll be watching this - would be nice to use in my 58503B which runs 24/7.

Qw3rtzuiop:
This guy already implemented an alternative display:
https://github.com/ragges/hp_display

TomT:
Hi Qw3rtzuiop,

Thanks for the heads up.  That project looks like an excellent option.  However, what I am proposing is a display that fits in place of the existing VFD, with a minimum of additional interface hardware.  It's going to be a tall order with punctuation and annunciators, but I am hoping that with the help of this crowd, we can make it happen.  With all the LCD replacements for CRT's showing up, I feel that retrofitting a VFD with LED's is a very viable option.

TheSteve:
There is no point to it if it doesn't fit in place of the original VFD.

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