Empty display

[ciccillover2]

I have an HD44780 compatible 4x20 LCD display, connected to the parallel port (LPT1 on 0x378, EPP setting in the BIOS, and switching to ECP doesn't make any difference). I am using this diagram:


When connected to the parallel port, as soon as I feed the +5v supply, it displays all bright pixels in the first and the third row, and as far as I know this should be a good sign. As soon as I start LCDSmartie 5.4, already configured for 4x20, LPT address $378 and the like, the whole display goes blank. Since a change takes place, this means to me that the port is responding, but the characters are not displayed. Already tried switching the multiplier to several values (2 to 8, i.e. $378,5), restarting LCDSmartie every time, to no avail. I have inpout32.dll in the LCDSmartie subdirectory. Am I missing something?

The display is not faulty, since it works fine on a Presario 700 under XP, same version of LCDSmartie. The computer on which I'm having troubles is built on an Intel D201GLY2 (Sis chipset). Any ideas?

[caesar]

Well, since moving the LCD to another computer and using the same LCDSmartie proves that it works, I suspect a problem in parallel port of the not working computer.

Either inpout32 is not installed (just having the dll is not enough, the driver must also be installed) or the parallel port is seriously undervolted (less than 2,8V signals).

[ciccillover2]

Thanks for the reply. Just to make sure, I have read that inpout32 works fine under Windows 7, can you confirm this?
Can you recommend one version that should work, since I could find many different ones?

[caesar]

ok, sorry for that!

Here are the signed files for Vista and 7 (x86 and x64)

http://www.highrez.co.uk/Downloads/InpOut32/default.htm

[ciccillover2]

Sorry for what? :p Thank you so much for your replies
By the way, it's the same version that I am using, but it doesn't work.
Supposing it's the LPT port signal that are not strong enough,as you suggested, how can I solve that? Has this already happened to anyone else? I'm searching but I can't find any useful information.

[ciccillover2]

The schematics I am using was not visible due to the huge size of the image. Thumbnail inserted.

[ciccillover2]

Similar problems with the parallel port only emitting signals at 3,3v are affecting a lot of people who try to use their CNC on their laptop. To to fix their issues, most of the time they add pull-up resistors to their circuitry, as I have read yesterday. Can't something similar be done with my display (or parallel port), too?

[caesar]

Hmm, never gave this a thought.
10K pull-up resistors should fix this if the parallel port can sink that current when needed (0,5mA at 5V). It's worth a try, resistors are cheap.

May be also that the port can't properly signal a logic low...

On the other hand, 3.3V is still in CMOS or TTL specs, so probably both the display and parallel port are at fault, one for signaling at 3.3 and the display for not registering it as a logic high.

[ciccillover2]

Ok, but, to be honest, I don't know what's meant for "pullup resistors". I can solder and I know what a resistor is. Where should I solder the resistors?

[caesar]

pull-up means a resistor between the data line and +5V (for each data line)
pull down means a resisotor between the data line and GND (also for each data line)

These are moments when some nice test equipment helps a lot but I guess that using a few resistors to test the theory isn't bad either.

[ciccillover2]

Your explanation is much clearer than Wikipedia's and many other's
Thank you, in fact trying won't take too long, nor be expensive. So can I solder directly o the display, rather than the motherboard, right?

New question: I was supposing this wouldn't be an issue, but my LPT cable is about 20 cm long. Do I need to shorten it?

[caesar]

20 is ok, the longest I used was 50cm of flat cable (like for the drives) and it still worked ok.

Before jumping to resistors, can you check the E and RW lines? At least the Enable line should hold a steady +5V if LCDsmartie is on. Hope you have a multimeter at hand!

[ciccillover2]

Got a multimeter, tried reading pins 1 and 14 (E and RW according to the diagram in the first post) directly from the parallel port, without connecting the display. They were both reading 3,3v at first, then dropped to about 0v after starting LCDSmartie...

[caesar]

at 3.3V, it's a good idea to try the pull up resistors
good luck!

[ciccillover2]

On those pins only? Or where else?

[caesar]

All the data pins need it.
D0-7, E, RW & RS

[ciccillover2]

Just to make sure, since the pull-up thing is new to me, I need to connect those lines to +5v through one 10k resistor for each line, right?
Like this:

+5v------10k------Data0
|
|
-----------10k------Datax
|
|
-----------10k------Data7
|
|
-----------10k------E
|
|
-----------10k------RW
|
|
-----------10k------RS

[caesar]

Sorry for the late response.

Yes, that is correct (while still keeping your current wiring)!

[ciccillover2]

A few days ago I decided to try the pullup resistors hack, but nothing had changed. So I tried connecting the display back to the old Presario and, to my surprise, it no longer works. I opened the LPT connector housing again and noticed one wire that is disconnected (but looked ok when I checked last time). It's pin 3 on the display. I can't figure out where it was soldered. Following the diagram at the first post, it should be on the center pin on the trimmer, but I noticed that the center pin is bridged to pin 15 on the display. I am a bit confused, and don't remember why I soldered it that way. I can confirm it used to work like that. Where should I solder pin 3 of the display?

[caesar]

Pin 3 of the display goes either to the middle of the 10k pot, or usually to GND for the majority of the displays. It's the contrast pin.

[ciccillover2]

Sorry for the late reply. Connecting or disconnecting pin 3 didn't make any difference, the display was still empty. However a few days ago I had to reinstall Win7 for a different reason, made some random attempts, then it started to work. Pin 3 is still disconnected and the resistors are still there but not fed with +5v. I don't know how come this works, and can't even remember what I did (nothing special anyway, just some fiddling with Smartie and InpOut), but it just works like this...
Thank you so much for your efforts, caesar