Multiple HD44780 on single LPT

Baldomero · Post by **Baldomero** » December 30th, 2005, 6:19 pm

Hello, I have a Hd44780 LCD connected to the parallel port, but also have lying around several LCDs.

I would like to connect more than one to the parallel port at the same time, is that posible?

the options are two 2*24 or four 2*16.

Post by **limbo** » December 30th, 2005, 7:00 pm

You mean to have a bunch of screens with the same messages?

Baldomero · Post by **Baldomero** » December 31st, 2005, 12:42 am

I mean, using two 2x20 as if a 4x20 LCD was. I remember doing that with a winamp plugin, but don't know if that is a relative standard feature of LCD PC software, so I am asking about LCD smartie.

Also, I was interested in knowing if more than two can be conected the same time.

Post by **limbo** » December 31st, 2005, 4:20 pm

I don't have expirience on two parallel connection on the same port....

The is an option to have more that one instance of the program running using different settings and as far as I know different addressing (serial or parallel), which means that using the same parallel port is not possible.

Baldomero · Post by **Baldomero** » December 31st, 2005, 8:39 pm

This winamp plugin is able to drive two HD44780 controllers with only one port parallel, that means you can have two LCD running at the same time, and so, have the double info with only one LPT.

The source code is available. I just post this because someone can be interested in looking at it

, and could be a good feature of LCD Smartie.

Hungry_Myst · Post by **Hungry_Myst** » January 4th, 2006, 9:05 pm

Would it be possible to offset the display on one 2*16 lcd by 2 lines so it looks like you are using one 4*16 lcd from the software's perspective?

Rafael_Brazil · Post by **Rafael_Brazil** » January 6th, 2006, 12:00 pm

Yes, you can... it?s like you use an 40x4 (this have two hd44780 controllers)

Hungry_Myst · Post by **Hungry_Myst** » January 6th, 2006, 5:40 pm

Nice, I'll try that when I get home

Baldomero · Post by **Baldomero** » January 6th, 2006, 6:10 pm

Then, any software that supports a 4x20 HD44780 LCD, supports two 2x20 wired this way? If this is true, then LCD Smartie supports it too?

Also, this combination is only applicable to this case, or can anyone combine two LCDs of any size, like making a 8x20 with two 4x20?

Rafael_Brazil · Post by **Rafael_Brazil** » January 7th, 2006, 6:23 pm

You can use with lcdsmartie...

For example... you have two 20x4 lcd?s...

You must set lcdsmartie to use 40x4...

I think that just works with two 20x4 because the 40x4 profile uses the enable2 pin...

Baldomero · Post by **Baldomero** » January 7th, 2006, 7:05 pm

Then, two 4x20 could work? I'll buy two of them, and try.

Rafael_Brazil · Post by **Rafael_Brazil** » January 7th, 2006, 8:54 pm

Yes, could work! So just set your lcdsmartie to 4x40....

Baldomero · Post by **Baldomero** » January 7th, 2006, 9:27 pm

But in this case, the result should be 8x20, not 4x40, shouldn't it?
It is like combining 2 2x20 to one 4x20:
2 X 4x20=8x20

Rafael_Brazil · Post by **Rafael_Brazil** » January 8th, 2006, 12:02 am

If exists a 8x40 display and lcdsmartie suport it... it?s true...

But the logic is that a 4x40 has two hd44780 controllers... the difference is the pin (enable 2).

ReverseEngineered · February 24th, 2006, 1:02 am

I know this is an old post, but hoped maybe I could still clear up some confusion.

This use of this second enable pin allows us to have a 4x40 display, which acts like two 2x40 displays (you can actually buy these 4x40 displays). What you are trying to do is to use two 2x20 displays as a 4x20, which is not at all the same thing.

The reason we use this trick is because the HD44780 controller chip only supports a total of 80 characters. To do 4x40 (160 characters) requires two chips. So one chip controls the top half, while the second chip controls the bottom half; as if they were seperate screens.

Now, if you use two 2x20 screens, you could use this to make a 4x20 screen, but LCDSmartie doesn't have an option for this. You would have to tell it that you are using a 4x40 display, and then you would find that LCDSmartie thinks you have a screen that is twice as wide as it really is. This probably wouldn't work very well.

It would be easy to change LCDSmartie to make this work, but since it's not really necessary (most people would just buy a 4x20 display in the first place), I can't see a reason to do so.

Aside from this, LCDSmartie doesn't have support for using two seperate displays (not combined into one large display, but acting as two seperate ones) on the same port. To do so would require a major change to the program! As limbo said, you should be able to run LCDSmartie multiple times (with different configuration files) and have it use two LCDs on different ports, but these would obviously act seperately, not like one large screen.

I hope this clears things up for you or anyone else interested in this idea.

MrMikey83 · Post by **MrMikey83** » March 4th, 2006, 4:05 am

I have had 4 IBM POS register display for several months. They are 2x20's with a display size of 1x5 inches so they are quite large. I decided to make a dual display cable from the schematic above and try it out.
I found that it works fine as long as I put it on 4x40 so that it activates both displays. Once it's going, I change it to 4x20 so that the text is only on the first 20 spaces to fit my displays. This works fine as the program doesn't disable the 2nd display until I restart LCD Smartie. I leave my PC on all the time so this isn't a problem unless I have to restart or the power goes out, then I have to pt it at 4x40, restart Smartie and change back to 4x20.

I wish I knew how to code plugins, I might try and make a tripple display (6x20).
~Mike

fishthecat · Post by **fishthecat** » March 23rd, 2006, 2:23 pm

ReverseEngineered wrote: snip...
It would be easy to change LCDSmartie to make this work, but since it's not really necessary (most people would just buy a 4x20 display in the first place), I can't see a reason to do so.

I've got a few 2 line LCDs lying around. If I could combine 2 into a 4 line screen off one parallel port, that would be ace.

Hope it gets added to Smartie.

fishthecat · Post by **fishthecat** » March 27th, 2006, 11:58 am

Actually, my LCDs are 2x24, and there's no 4x24 setting in Smartie right?
Is there a way I can get 4 lines on them?
I'd be OK if I could get a centred 4x20 over my 2 2x24 LCDs.
Does anyone know if that might work?

Post by **caesar** » March 27th, 2006, 9:53 pm

I have used before two 2x24 displays as one (set smartie on 4x40).

You just have to setup your screens so that info displayed is 24 characters wide or it won't get shown because of the display dimensions, the driver chip processes the info as it can handle it (2x40 per hd44780) it just isn't displayed.

No problem!

Go 4 it!

PS: when you use 2 displays, even two 2x16, always set smartie for 4x40 as 4x20 is another story! And you can't use two 4x20 either!

fishthecat · Post by **fishthecat** » March 27th, 2006, 11:35 pm

ceasar wrote: I have used before two 2x24 displays as one (set smartie on 4x40).

You just have to setup your screens so that info displayed is 24 characters wide or it won't get shown because of the display dimensions, the driver chip processes the info as it can handle it (2x40 per hd44780) it just isn't displayed.

No problem!

Go 4 it!

PS: when you use 2 displays, even two 2x16, always set smartie for 4x40 as 4x20 is another story! And you can't use two 4x20 either!

Thanks. I'll give it a go. Should I use the same wiring as in the previously posted diagram?

Rafael_Brazil wrote:Yes, you can... it?s like you use an 40x4 (this have two hd44780 controllers)

Post by **caesar** » March 28th, 2006, 6:11 am

That wiring is fine if you have two identical lcd's but you shold wire the contrast separately. Also the backlight should be wired separately, the current can get even close to 1 Ampere depending on backlight type so separate potentiometers should be wired there too.

It is best to check your display's datasheet for the variable resistors values but 10K Ohm for contrast and 100 Ohm for backlight are fine values to start with (conservative), you won't kill a display.