Difference between LVDS and TTL?

bradf0rd · 10-27-2002, 08:45 PM

As the title says, what exactly is the difference? And will it affect the choice of controller for a given screen?

Thanks

MP3DUB · 10-28-2002, 11:02 AM

Just different types of logic. Generally, Transistor-Transistor Logic is older, and Low Voltage Differential Signaling is newer. Someone with EE knowledge could tell you more.

eeguru · 11-02-2002, 02:42 PM

TTL is a zero or ~+5 volt binary one or a zero. In LCD applications, it means most of the signaling is trasmitted in parallel and measured against a common signal ground. This has two bad implications: 1) a lot of wires needed to transmit the signal (6-12 lines per color plus a dozen signalling lines), and 2) it's very succeptable to interferance and voltage falloff limiting the transmission distance to a couple feet at most. You typically see TTL LCD connectors as a 34, 40, or 44 pin 2mm header.

LVDS is Low Voltage Differential Signalling. Instead of the presence of a voltage or absense of voltage relative to a common signal ground to project a binary 1 or 0, it uses 2 wires per signal with a + and - voltage difference. If both wires are at equal voltage, its a zero. If there is a measurable voltage difference, its a one. Same amount of potential and thus current flows on each wire but in opposite directions dampening interferance. It also operates at a higher clock frequency allowing multiple signals to be serialized into fewer wires. It can be transmitted up to 25 meters from the source. This typically will have a 20 or 24 pin surface mount DFP or DVI connector.

bradf0rd · 11-02-2002, 02:55 PM

So basically, the two formats are completely incompatible (ie a controller that works with a TTL screen has no chance of working with LVDS?)
And welcome eeguru!!

Nice to see people join a forum and the first thing they do is answer some questions

zootjeff · 11-02-2002, 08:06 PM

Quote:

Originally posted by eeguru
If both wires are at equal voltage, its a zero.

Are you sure about this? I thought differential ment they are always opposite polarity..

eeguru · 11-02-2002, 08:34 PM

Quote:

Originally posted by zootjeff
Are you sure about this? I thought differential ment they are always opposite polarity..

In a one state, they are at opposite polarity. In a zero state they are equal. I say equal because they can be at a floating voltage and still register a zero if floating at the same voltage (what differential is all about).

eeguru · 11-02-2002, 08:43 PM

There are converters out there. I'm using the Advantech PCM-3540 transmitter and receiver pair (about $60 a peice) and a 12m cable to run between the trunk mounted CPU and the in dash LCD. They support both TFT & STDN, +5V or +3.3V LCDs. The transmitter is PC/104, but has the option of getting it power from the 104 bus or the LCD cable (so it can be used standalone). It also has the option of powering the reciever remotely through the DFP cable; although you still have to supply the +12V backlight inverter power at the recieving end.

A lot of VGA cards are coming with DVI (LVDS) interfaces now and you can readily buy a DVI to DVP cable that will work with Advantech's receiver. The receiver has a mini (floppy drive) type power connector on it for supplying +5 and +12V which will also power the LCD if it has an inverter integrated. Kinda nice to power everything off standard PC power supply connectors.

zootjeff · 11-02-2002, 11:22 PM

Quote:

Originally posted by eeguru
In a one state, they are at opposite polarity. In a zero state they are equal.

Hmm, I think you might want to take another look at some data sheets at a differential timing diagram. What you described is not a differential signal, both the positive and negative drivers change polarities in the transition from a high and low state. The state (1 or 0) just reflects the voltage of the + terminal. Things like USB have states when both are equal, but that means something else like Idle or something. If they are both different and then they are both the same, say A is 1 volt, B is 2 volts and then they both go to 1.5 volts it almost works, but the input to differential amps don’t work this way. If A is 1 volt, B is 2 volts, then they both go to 1 volts it isn’t differential because A didn’t do anything. To be a differential signal, they both have to transition about a common voltage (called the common mode.) They have to transition an equal amount in opposite directions such that the difference is always the same about the center voltage. The state where they are both the same doesn’t mean a 1 or 0. Like I said, 1 and 0 is determined by the plus terminal, the minus just does the opposite.

The main advantage for using differential signaling is that it exibits "common mode rejection" if you induce a current onto a differential line, because they twist the pairs both will be affected the same ammount and the net will be zero because the reciever subtracts one from the other to see what state it is.
IE A+ is 1 volt, A- is 2 volts, this is a 0 state.
A+ is 2 volts A- is 1 volt, this is a 1 state.
The reciever takes A+ minus A- if it is a negitive voltage, that is how it tells a 0 state. If A+ minus A- is positive, that is a 1 State.

So if you take a 5 volt spike and put it on the line, A+ is 6 volts and A- is 7 volts, A+ minus A- is still negative and the same ammout negative as before. This is how common mode rejection works and it is why you use differential signaling. LVDS is just lower voltage swings then was commonly use before it was introduced. LVDS is like +- 100 mv across 1.25 volts common mode.

zootjeff · 11-02-2002, 11:52 PM

Quote:

Originally posted by bradf0rd
As the title says, what exactly is the difference? And will it affect the choice of controller for a given screen?

Thanks

I think the answer you are looking for is, Yes it will affect the choice and no they are not compatable, but they can be converted:

LCDs that take in LVDS, use a serial encoding scheme, have fewer wires and require that the Driver card puts out LVDS.

LCDs that take in TTL have more wires but have to be closer to the controler card LVDS and TTL are not compatable

You can make a converter from TTL to LVDS if you buy a TTL to LVDS converter chip. TI and National semiconductor make them. They are in tiny packages so it will probably be a pain to do it.

Check out these data sheets for more details than you probably care about.

http://www.national.com/pf/DS/DS90CR214.html
http://www.national.com/pf/DS/DS90CR215.html

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