# How does a Line Driver actually works?

So I've searched for days for information about the 74HC244 line driver. As always, the Datasheet provides raw and not very good information about the actual component :) (I mean, it gives us the temperatures, but not how the actual component works...)

Any way, there is very little information about Line Drivers in general. So, how do thy actually works? And how the 74244 can act as a line driver? All I see in the inner structure is enables and input to outpuut wires.

EDIT

I'm using the 74244 to connect between my Altera (FPGA) and a servo motor. I think that in order to reduce the current that the FPGA suplies to the servo, I need to put the 74244 in between them, thus reducing the "effort" from the Altera. Is that necessary? Or the Altera can provide a good current flow to the servo without damaging it's abilities?

• That datasheet gives everything you need to actually use a 74HC244. It sounds like you're looking for a equivalent circuit. – Connor Wolf Oct 20 '14 at 11:15
• In case it's the apparent lack of power to the chip that's confusing you: The functional diagram on that datasheet omits the power pins, but they are there and must be powered for the part to function. That power is used by each buffer element to increase the load it can drive. – alx9r Oct 20 '14 at 18:12

A line driver is simply a buffer. What you put in one end comes out the other. However, it is typically able to sink and source much more current than, say, a normal GPIO pin on an MCU.

The increased current is able to overcome the capacitance caused by having many devices on a bus.

You can think of it as the logic gate equivalent of an amplifier.

• But how it works? I mean, how the 74244 can increase the current with just sending the input to the output? – Eminem Oct 20 '14 at 11:18
• @Eminem - What makes you think the input is connected directly to the output? What do you think a buffer is? – Connor Wolf Oct 20 '14 at 11:20
• @Eminem Language barrier here, I think. When I say "What you put in one end comes out the other" I refer to logic values (HIGH or LOW), not currents and voltages. You put a HIGH on the input you get a HIGH on the output, but with the potential for more current than what's sending the original HIGH. – Majenko Oct 20 '14 at 11:27

I'm not sure what you mean by "how it works". The datasheet tells you pretty much everything you need to actually use the part, though NXP does tend to be a bit sparse on internal details.

Anyways, if you look around a bit, you can find some slightly better datasheets that have more detailed internal diagrams.

The datasheet also states: Chip Complexity: 136 FETs or 34 Equivalent Gates, which tells you that (at least ON Semiconductor's implementation of the 74HC244) the actual chip uses a combination of 136 transistors.

If you want more detail, you should start reading about how transistors work, and how they're used to make the logic gates in the shown diagram.

There are two principal functions of this line driver.

The first is to provide a power gain. The circuits that are driving the inputs may not have the power to drive the impedance of a longer wire length. This chip does not perform any logical function; it is just giving the signal a strength boost. The line driver also serves to isolate the local circuits from electrostatic discharge from connections that might go off the board.

The second functionality is that it has a tristate output, which means that when the enable pin is not asserted the chip does not drive the output signal at all. It's as if the chip were disconnected from the wire. This allows you to have multiple line drivers on the same wire, and as long as they properly take turns driving it they can share the single connection. Tristate mode also saves power by not driving the line when there is nothing to send.

It should be noted that the device identified above is a "tri-state" driver. This means that it can set the line "high", "low", or can effectively disconnect itself from the line. This is used in situations where any of several components can "drive" the bus (though hopefully not more than one at a time).

It can be used, eg, for a "bi-directional" bus between processor and memory or between processor and devices: The processor, when control lines are set in one state, can output data onto the bus to be read by the memory controller or I/O device controllers, and when the control lines are in another state the processor "listens" on the bus and the memory or device controller "drives" the bus.

How does a line driver actually work?

An alternative/equivalent (has detailed transistor schematic) is the AM26LS31 specification / datasheet. TI Texas Instruments RS422 line driver Description:

The AM26LS31 device is a quadruple complementary-output line driver designed to meet the requirements of ANSI TIA/EIA-422-B and … The 3-state outputs have high-current capability for driving balanced lines such as twisted-pair or parallel-wire transmission lines, and they are in the high-impedance state in the power-off condition. The enable function is common to all four drivers and offers the choice of an active-high or active-low enable (G, G) input. Low-power Schottky circuitry reduces power consumption without sacrificing speed.