Problems with 16 x 2 LCD panel

[benb2000]

I am having a problem in displaying anything on the LCD. I spent a lot of time setting the unit up, checking that all the pins and cabling is correct. The LCD has a 14 pin header, with pin #3 supplying the power to change the contrast of the LCD. This voltage is supposed to be adjustable by the small pot on the PLC ( Contrast Adjust ), a voltmeter on pin 3 indicates this pin is very close to ground potential, no matter the pot position, Looking at the PLC Board the pot has three pins, one connected to Pin #3 of the 14 pin header, one to pin #1 of the 14 pin header ( Ground ), the other, should be connected to +5.0 vdc, I could not see where the PCB trace went to, This pin ( on the PC Pot ) has no voltage on it when the unit is powered up. Running the demo programs show the PLC is functioning, but no info is shown on the LCD. Oscilloscope probe shows pulses on all pins but pin 5. So it would indicate that either the contrast is turned off or the new LCD is toast.

The LCD is a brand new Optrex DMC-16230NYJ-LY-CGE-EB unit purchased from Digikey

Any assistance would be appreciated

[support]

The DMC16230 is not supplied by us. However, it should work with our LCD port without any issue if the wiring is done correctly. Please make sure that you did not mistakenly swap pin 1 with pin 2, pin 3 with pin 4 and so on (a common mistake). ?

Pin 1 : 0V
Pin 2 : +5V
pin 3 : constrast signal (via variable resistor to 0V)
pin 4 : control signal (TTL)
pin 5 : 0V
pin 6 : control signal (TTL)
pin 7 to 14: data bus (TTL)

Note that wrong connection of signal may damage the LCD or the driver IC on the PLC. If you have one of our LCD216 or LCD420 you can use it to verify if the PLC's LCD driver is still working properly. ?Using our supplied LCD216 and LCD420 is much easier since we supply the proper cable and such issue would not arise. ?However, if you need to connect your own LCD display bought from the open market then you do have to take the time to figure out the proper wiring. Good luck! ?

[benb2000]

Thankyou for the reply. What should the voltage range , via the contrast potentiometer, be on pin3. Typical LCD litrature indicates 0 to +5 vdc. Is the Contrast potentiometer set so that one leg should have +5.0 vdc, one leg to ground, and the wiper to pin #3 of the LCD 14 pin header? Is a schematic available for the LCD port? I am currently not seeing any voltage on one leg of the contrast pot. Is there an IC or transistor used to switch power to the contrast pot?

[benb2000]

As it turns out, the Optrex DMC-16230NYj-LY-CGE-EB, being an extended temperature unit, requires a -2.5 volt level on pin #3 ( Contrast Voltage ) of the 14 pin header. We are now functioning properly.

The T100MD-888+ that we purchased, was purchased with an Industrial temperature specification of -20 Deg. C. For Low Temp operation of LCD's, it would appear that a negative contrast voltage is required. Is there any negative voltage available on the T100MD-888+?

[support]

Unfortunately there isn't a negative voltage available on the MD888+ to power the constrast pin of your extended temperature version of LCD. This was not an issue with the standard LCD216 and LCD420 that we have been selling.  

One method that I can think of and you may like to try is to connect the 0V of the PLC power terminal via three series diode before returning to 0V of the power supply. In that case, the 0V of the power supply will appear to be at -2.1V with respect to the 0V of the PLC. You can therefore use the 0V of the power supply to connect to constrast pin to achieve your negative voltage.  Theoretically it should work but you do need to experiment with it just to be sure:

PLC                                                                                                                                        Power Supply

   0V --|>|----|>|-----|>|------- 0V

[benb2000]

This may be a valid approach, but you will still have to break the ribbon cable line to pin #3, in order to attach it to the -2.1 volt return line. I am using +12 volts for system voltage, ( mobile application ) and have ellected to go with a charge pumped inverter and a temperature compensated LCD bias supply. Thankyou for your assistance.