LCD module ribbon cable, How Long Can It Be ?

[rizzy_dascal]

Hi,

I would like to make my project separate from the display panel, does anybody know how long the ribbon cable can be ? , can I run in to problems if it is too long ? , Do I need to amplify the signals somehow ? , I only need to make it about 12 inches really.....

Thanks In Advance
Rizzy

 

[jiripolivka]

LCDs have almost no current consumption, so your cable can be as long as needed. Only use a screened cable to avoid induction interference from other AC power devices. Check the signal frequency on display lines; if the frequency exceeds one MHz, then cable capacitance between lines and ground (screening foil) may limit the cable length; this depends on source resistance.
I would try to cut the ribbon cable into 5-meter lengths and try to connect one, two, three, until you see some smearing or other problems on the display. 12 inches must go well anyhow.

 

[Ice-Tea]

That really depends on the type of display, and more importantly on the speed of the signals on it. If the length of the cable represents a delay of, say 0.1 x Frequency or more you might be in trouble, especially for flank-sensitive or asynchronous signals.

Actually, the correct way of phrasing is 0.1-0.2 x Tr but looking at the frequency is often a good approximation

 

[FvM]

That really depends on the type of display

I presume that "LCD module" means a standard HD44780 controller parallel interface.

My opinion is that 12" or so can work, but the interface has about to no ESD or RF interference strength unless operated in a metal enclosure. In terms of professional design, it's likely to fail all relevant EMC tests.

 

[rizzy_dascal]

Hi Guys,

Thanks for your replies, yes it is a standard 4 x 20 lcd module, the unit is a switching timeclock using a pic 16f877A chip, all this will be inside a steel enclosure with heating contactors in etc, however i would like to have a metal panel on the front of the enclosure with the lcd on it...

Thanks to all
Rizzy

 

[FvM]

Contactor arcing might be a problem.

 

[pjmelect]

LCD module ribbon cable, How Long Can It Be ?

You can't answer a question like this, it is like asking how long is a piece of string. The limiting factor is the capacitance between the signal wires and how the controlling program was written. If the controlling program was written for long cable lengths then there maybe no reasonable upper limit on the length of the cable, however if the controlling program was written so that the timings are at their minimum then even 12" maybe to long. If you have access to the program source then ensuring that all of the LCD timings are generous, then 12" is not an unreasonable length.

 

[rizzy_dascal]

Thanks for the extra info guys, if the contactor interfered with the LCD is there an easy way to stop it, could you shield the ribbon cable with foil and earth it or something ?

cheers all again
Rizzy

 

[pjmelect]

Shielding the ribbon cable would only help if you have interference problem and will in fact make the capacitance problem worse. Maybe termination resistors would help, but changing the software would be better. Try it and see.

 

[FvM]

if the contactor interfered with the LCD is there an easy way to stop it, could you shield the ribbon cable with foil and earth it or something ?

The interferences can cause false commands to the LCD, resulting in garbage characters or even loss of configuration. Screening and earthing can improve the interference strength but may be insufficient. One reason is that the LCD board itself, e.g. the display metal frame has a stray capacitance against earth and picks up common mode interferences that couple into the parallel interface.

My conclusion is to strictly avoid off-board parallel interfaces and use e.g. displays with serial interface. But it's difficult to foresee the interference strength in a specific design.

You can go the other way, place RC snubbers or varistors across arcing contacts. Or make the display software fail-safe by repeatedly checking the LCD controller's health state (read access required) or refresh the configuration quite often.

 

[Chris1990]

my mate ran a little 4x20 display with about 75cm of cable between it and the driving source.. only issue he had was the power occassionally dropped and the screen blanked.. it was avoided by using a capacitor across lcd power and ground, and a diode on the lcd power to stop it backpowering the driving source.

Hope this helps you!

 

[FANT]

I have used the cable up to 1 meter, of course shielded.
Only caution a capacitor of 10 uF between the supply pin of the display to avoid voltage drops or spikes.

Made about 1000 pieces without any problem.

Mandi

 

[rizzy_dascal]

Hi guys,

Once again thank you for all your interesting comments

Rizzy

 

[rizzy_dascal]

Hi Guys,

Just a thought but would I be better using an I2c LCD module, are they less susceptible to interference ?

cheers
Rizzy

 

[Chris1990]

it would depend on the speed of communication.. if, by switching to i2c, the bit rate was reduced, then it would allow you a greater noise margin.. theoretically.. you could convert the i2c into RS485 and then have a hundred metres or more between the two..
problem is, when you have clock signals etc, you generate more noise. etc.. but this is potentially seen with your current display method.
perhaps this was a bit waffly but i hope you get the meaning!