Identifying LCD Display - VDO Logic Combi

[JohnW]

I have an instrument (A VDO Logic Combi) used for engine monitoring on boats. The LCD is water damaged, and I am assuming that it is a 16 x 1 display. It displays normal upper and lower alphabetic characters, decimal point, and uses block graphics to indicate fuel level via a bar. The LCD has a metal outer case 77mm x 22.5mm x8mm, thats just over 3" x 7/8" x3/8" and metal case has 6 lugs used to locate the case on the PCB (twisted to secure it). I have not yet attempted to remove it, but the only connections appear to be a ribbon type cable with two conductors at each end. It is located on the track side of the board.

On the upper side of the board is an PCF8578T display driver IC with the Phillips logo.

Can anyone identify the LCD (black on white), and confirm the connections? I assume it must be a serial interface.

VDO no longer support the instrument, current in 1995, and now out of production for 15 years or so.

 

[kellys_eye]

Please show us the display and connections to it.

It may be a serial-programmable display. There are many 16x1 LCD panels out there but the interface has to match (of course).

http://www.ebay.co.uk/itm/5V-Negati...076127?hash=item465b46c81f:g:h60AAOSwa~BYY2U~

That said, the PCF8578 is an I2C capable device and there are I2C interface boards available for the LCD I linked to above.

http://www.ebay.co.uk/itm/2Pcs-IIC-...922126?hash=item33a2c17f4e:g:nRkAAOSwFTRTpUVI

Might not be an 'exact' physical fit but you may be able to cobble something around it?

Google "LCD Smartie" too as there are many reports on all sorts of re-usable LCD displays and there may be information on yours.

 

[JohnW]

Hi Kellys_eye

Will look at your links, thank you very much for the pointers.

I am uploading a photo. It shows the front three lugs through the board. It is now quite mobile, so I am assuming the connections are some sort of ribbon cable. There are just two pcb lands connected at either end of the display.

 

[kellys_eye]

If one end has two pins and the other 8 or 10 then it sounds to be a fairly common type and should be replaceable.

Can we see the reverse of the board (where the pins solder through?).

Note that the display is usually a one-piece device and that the metal 'bezel' should be considered an integral part - you will be looking to replace the display as a complete module rather than just the LCD bit within.

 

[JohnW]

It seems to have 2 connections at each end - perhaps VCC and GND, plus data and a clock signal? I am surmising, as I don't know anything about LCD variants.

I have not removed it yet because I left my soldering iron behind on the boat. I am waiting for a faulty but otherwise reasonable condition unit to come from Australia, on which the LCD is still working.

However, you can see clear daylight between the loosened metal case (and LCDpanel) and the PCB on which it is mounted, ruling out any other connections.

 

[JohnW]

OK, here are some photos. Orange paper proves its fresh air underneath, not connections. The two ends are shown, and the reverse shows the controller on the PCB, and the two pairs of two lands at each end that seem to be the only connections.

Can the controller PCF8578T output i2c direct to the LCD?

 

[kellys_eye]

The LED obviously needs sorting - it's the backlight for the display, naturally.

The PCF8578 is the display controller and addresses the rows/columns of the LCD and the thru-hole board connections adjacent to the chip would be the normal means of connection. I'd have guessed that the thru-holes led to pads that themselves were part of a zebra-strip type connection between the board and display.

The four holes to the far left would be, I guess, the serial data from whatever is sending the data.

It's unusual to see the actual LCD as a part of the whole PCB assembly - they are normally sub-assemblies and replaceable as 'one'. If you can lever the metal frame off the LCD itself it might resolve some irregularities.....

 

[JohnW]

LED's light the tachometer that is part of the unit. This LCD is still working in a sense, bar the water damage. It displays the first three characters or so perfectly, and some of the rest is guesswork.

 

[davenn]

LED's light the tachometer that is part of the unit.

the LED is the display backlight, as KE said

This LCD is still working in a sense, bar the water damage.

the display itself is damaged and not just by water corrosion .... that big black patch on the display is where the liquid crystal has failed

It's unusual to see the actual LCD as a part of the whole PCB assembly

reasonably typical actually

I really don't know what you were trying to prove by putting paper strips in there ??

 

[JohnW]

The paper strips demonstrate that there are no connections to the rear of the unit, just to the two ends...

 

[kellys_eye]

The paper strip test is what's puzzling me.

The two terminations at one end may be the back light, the other two 'could' be a serial interface but the PCF8578 actually IS the display driver device for the LCD itself:

http://pdf1.alldatasheet.com/datasheet-pdf/view/18218/PHILIPS/PCF8578.html

consequently there MUST be a connection between the PCF.... chip and the LCD to make the row/column connections - this cannot be done on only two wires.....

Yes, the PCF device uses I2C but that's the connection between the PCF and the processor that sends the signals to be displayed.

So how does the PCF device connect to the LCD??????

 

[davenn]

The paper strips demonstrate that there are no connections to the rear of the unit, just to the two ends...

consequently there MUST be a connection between the PCF.... chip and the LCD to make the row/column connections - this cannot be done on only two wires.....

agreed

John, you really need to remove the display from the board so we can see the board and the underside of the display

 

[JohnW]

OK, you'll need to be patient. I'll put an order in to RS for the soldering equipment!

I am as puzzled as you are. There are a row of connections to/from the PCF (12 + 4) shown on the photo of the backside, but these are through holes, not soldered. I can see daylight on the other side of these - hence orange paper demo.

I'll come back when I have progress to report.

 

[JohnW]

Well, as promised, I'm back with more photos.

Sorry for not believing you, but the connections at the ends were for two LED's, one at each end, providing the backlight for the display. Or they might if they were not in very bad condition/broken!

The connections for the LCD are taken out from the controller to a series of pads on the upper side of the board. These are connected by a kind of rubber pad that is in compression between the LCD in the metal frame, and the pads on the PCB. In one of the photos you may just be able to discern distinct conductors within the pad, and you may also be able to see the pads on the edge of the reverse side of the LCD. Hence no soldered connections.

The unit from Australia arrived, and I removed the LCD from that, along with the LED's. However, when powered up, the replacement LCD was showing gobbledegook - graphics characters but only on the rhs, the lhs being all black. I had noted that the PCB layout of the LCD controller, in particular, was very different on the Aussie unit.

I have reverted to the naff LCD from my unit, which displays something, but I am not much further forward, but I do have a more waterproof case.

Presumably the firmware/software and graphics on the Aussie unit are different. Is there any chance I might be able to re-program the LCD?

 

[kellys_eye]

I'd have guessed that the thru-holes led to pads that themselves were part of a zebra-strip type connection between the board and display.

The 'strip' between display and circuit board is known as a 'zebra strip' and consists of multiple conductive strips sandwiched between insulators and is commonly used to make the connections to LCDs like yours.

They do tend to rely heavily on 'pressure' to maintain contact and equaly, the contact surfaces need to be scrupulously clean AND the pressure needs to be evenly spred across the width of the display.

You must get that 'crap' off the board too - check for continuity of the thru-holes that are obviously corroded.. you might have to solder a wire through to make the conection if the hole has failed.

The metal bracket gives the mechanical force to assure this connection so needs to be properly located - the tabs appropriately 'bent' etc. EVENLY.

If the LCD replacement is an identical device then the 'failure' is the zebra strip and/or its connections.

 

[kellys_eye]

Addendum - since you managed to get a paper strip under the display (?!?) you obviously managed to pass it under the zebra strip - indicating that the pressure was poor to say the least.....

 

[JohnW]

Thanks very much for your input, KE. I am sorry I did not pick up on the 'zebra' as I had not come across this before.

I appreciate now the need for a clean surface and firm pressure. I had loosened the casing and in fact it was only held in position by the back light diodes at each end, hence the ability to get the orange paper in.

I am confident that the replacement LCD from Aussie was connected well: it displayed characters and responded to the roll-over of functions (mode), but not meaningful ones. In some way the different layout of the PCB drive lines from the controller must reflect a difference in the LCD, although I find this surprising.

Re-fitting the original LCD resulted in its limited but otherwise normal functionality.

The Aussie unit as a whole was wrecked by the discharge of a high voltage through it, but the LCD was operating normally.

I'll have another look at the different layouts of the controller, and test the Aussie again with its own controller.

Any further thoughts would be most welcome.

John

 

[kellys_eye]

Does the LCD have any markings on it? With the same segment/dot layout it's unusual to find matching displays having differing connections.... by holding the display(s) at an angle you might see the 'elements' that make the display up but if there is ANY difference between them (extra characters, added lines, different symbols etc) then the pin-out is quite likely to be different too.

Another thing to be wary of are the connections on the LCD itself - they are vacuum deposition connections onto the glass substrate and VERY easy to damage/destroy etc. Very close inspection and an ability to recognise what is good from what is bad (in terms of connection surfaces) is required but since you have two displays you should be able to make comparisons to see any significant damage that may/may not have occurred.

With the display running, apply LIGHT pressure along the zebra-stripped edge and see if any additional characters start showing - this is a good indication of poor zebra strip conductivity and the need for further cleaning and/or adjustment of the clips holding it in place.

 

[JohnW]

Hi again,

The only marking on the 'new' LCD is a small red and a grey dot on one edge. Not very helpful. I cannot see any structure within, some small black dots on the top edge at each side.

I've refitted the 'new' LCD into its corresponding mother board, and powered it up. It displays 'Alarm' and 'Failure' nicely. I have had no problem re-assembling the LCD into the containing bracket, and so I am disinclined to believe that my trial last night with this LCD in my motherboard was not validly connected. But the left hand side of the display was solid black, and the right hand side graphical nonsense.

I have traced the initial lines from pin 56 (R0) through to circa R7 to the top right hand edge of the LCD on both boards. The 'new' motherboard is of much better quality, but carries an older (lower) serial number with more i.c.'s - the layout and population being very different.

I think this may be the end of the line, unless I can find an identical LCD?

 

[kellys_eye]

If you're 100% confident that you have removed all corrosion and that there is no damage caused by it (can you see UNDER the control IC?) then the controller may have been damaged by the original corrosion.

Swapping the control chips is possible but requires particular skills and care. If the equipment is valuable enough to consider it then give it a go or find someone locally who is competent enough to achieve this.