7 Segment LCD Display Common Pins

Question

I have these LCDs "VI-201-DP-RC-S" & "LCD-S401C52TR" This is the first time that I am using LCD displays. When I placed the leads of my multimeter between the common pins, I didn’t measure 0ohm like in a seven-segment display but in (VI-201-DP-RC-S) I measured 2.87Kohm and in (LCD-S401C52TR) I measured 3.54Kohm. Why I don’t measure 0ohm like on LED 7 segment displays?
Also, on both LCDs if I only connect one common pin the LCD display all the digits displayed fine and if I connect and the two common pins together doesn’t change nothing at all for example in contrast, viewing angle etc. at all. Do you know why there two common pins... because it works fine with one? Οn seven segment display I believe there two common pins (also a seven segment display works fine with when only one common pin is connected) maybe for current reasons but in LCDs the current is imperceptible.

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Answer 1

The common in a static LCD display is a transparent yet conductive ITO (Indium Tin Oxide) layer on the inner layer of the top glass. It's made thin so that it won't block more light than necessary. Typical surface resistivity numbers on the net vary over a wide range (from 10Ω/□しろいしかく to 1000Ω/□しろいしかく). Image below from here

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Possibly under some drive conditions it makes a slight difference whether you connect to one or all. I don't see any reason not to connect them together.

• Chester Gillon: The VI-201 datasheet excerpt in the question has REMARKS : PCB LINKAGE : PIN1 and PIN18 which suggests both COM pins should be connected together on the PCB (for best performance?).
• Spehro 'speff' Pefhany: @ChesterGillon Yes, agreed.

Answer 2

There is a remark on the datasheet of the VI-201-DP-RC-S that says: "PCB Linkage: Pin1 and Pin18".

This means that each COM pin belongs to one digit and the COM pins are not connected on the LCD display.

This makes sense, because you might want to drive both digits completely individually, i.e. without interference from one to the other. This is required in multiplexing of digits: You can combine the segments driver signals from both (or more) digits and activate the display with the COM signal from the digit that you want to show at the multiplexing time slot.

• DarkKnight: If the common pins on the "VI-201-DP-RC-S" are belong seperatly to each digit the multimeter shouldn't show OL instead of 2.87Kohm?
• Stefan Wyss: The LCD has a (pretty low) surface resistivity which means that the pins are not well isolated. That's what you are measuring.
• DarkKnight: I tested on the breadbord the LCD VI-201-DP-RC-S and if only I connect one of the two common pins both digits are displayed, so each common is not for multiplexing each digit. I beleive something else is happening. Thanks for your replies.

Answer 3

These are LCD segment(or glass) display, they are not wired like 7-Segment LED. The table on the datasheet provided the information on how the segments are wired in related to which COM pin.

You can't use a direct DC to drive the segment like the way you drive a 7-segment LED for segment LCD. They need to be biased and multiplexed based on the information provided by the display vendor, the datasheet that you show lack of the information of bias voltage, but likely as 1/2 bias or 1/3 bias.

To drive the display, it is a little bit like charlieplexing, take com1 as an example, depend on how the com1 pin is polarized, certain segment will be turned on, flipping the polarity, the segment that was previously on, will be off, and the other segment that was previously off, now will be on, so to display a digit, it needs to multiplex each of the com pins twice with different polarity and go through all the four com pins just to display one digit.

You can see my blog on How to drive a segment lcd glass for more insight.