User`s manual

RabbitFLEX User’s Manual www.rabbit.com 59
5.4 Digital Outputs
There are several types of options available for digital outputs. There are sinking and sourcing drivers, as
well as line drivers. No matter which digital output type you have, the circuit is activated by a call to
flexDigOut() or flexDigOutGroup16().
5.4.1 Sinking and Sourcing Drivers
One approach to boost the power of DC control signals is to use a sinking output. There are two choices
for sinking outputs on a RabbitFLEX BL300F: a sinking output that can sink up to 1 A and one that can
sink up to 100 mA. Another advantage of a sinking output driver is that it can control large voltage—up to
the rating of the transistor used. The 1 A sinking driver can control up 40 V.
A sinking output uses an NPN transistor. The transistor’s emitter is connected to ground potential. When
on (the control signal, called the base, is high), the output terminal, called the collector, is connected to
ground.
Another type of digital output driver is the sourcing output driver. It is the complement of the sinking out-
put and uses a PNP transistor. The emitter is connected to a positive supply. When the transistor is on (a
low voltage on the control signal), the collector is connected to the positive supply.
As with the sinking driver, there are two choices for sourcing outputs on a RabbitFLEX BL300F: a sourc-
ing ouput that can source up to 400 mA and one that can source up to 50 mA.
At first, sourcing drivers seem to make a little more sense than sinking drivers, but they are not as efficient
as sinking drivers. PNP transistors typically do not have current ratings as high as their NPN counterparts.
Sourcing drivers can, however, source current to devices that are connected to negative power supplies.
One way to “see” the difference between sourcing and sinking power is to expand on the water analogy
that is often used to understand the different terms that describe electricity, such as voltage and current.
Using a water tower, Figure 5.2 illustrates that the transistor type used in the sourcing driver is connected
to the power supply (i.e., the water in the tower), thus sourcing power for the attached load when the tran-
sistor’s control line is activated. The transistor type used in a sinking driver is connected to ground (i.e.,
the water at ground level) thus causing current to flow in the attached load by sinking the current to ground
when the transistor’s control line is activated.
When selecting a driver, make sure to consider the type of load you will be driving. If driving an inductive
load, such as a relay, solenoid or electric motor, select a driver that has diode protection to prevent burning
out the driver transistor. See Section 5.4.3 for more on protection diodes.