SN74AC244 Buffer Chip Used on Gertboard

The SN74AC244 is an octal buffer / driver IC with tri-state outputs, which can be either 1, 0 or high impedance. The Gertboard, which is the Raspberry Pi expansion board, uses three of these ICs to provide manually selectable bi-directional buffers for 12 of the Raspberry PI GPIO pins. The octal buffer driver is one of those wonderful components that all electronics and computer students learn about very early because this is a vital component for computer data buses.

Each side of the IC has four buffer circuits, which feed their outputs to the pins on the opposite side. This pin configuration is useful in bi-directional bus designs, where each line has a pair of buffers for data travelling in opposing directions. The diagram above shows that when pin 1 is tied to ground (0 V), the control signal makes the buffers go into an active state. An input signal at pin 4 passes through the buffer and emerges at the output pin 16.

Output Enable

The Output Enable (OE) control signal is fed through the pin 1 (1G) and controls four of the buffers together. The OE pin 19 (2G) controls the four buffers pointing in the opposite direction.

The control signal influences the buffer output characteristics because when enabled, the buffer passes the data through, however, when disabled the output goes into a high impedance state and the data does not pass through. Thus, the outputs may have three different states of either 1, 0, or high impedance.

The OE signals are active low, which means that when pin 1 and pin 19 are tied to ground (0 V) the buffers will pass the data through. However, if tied to +5 V, then their outputs go into a high impedance state. On the Gertboard, pin 1 and pin 19 of the SN74AC244 IC connect to ground and therefore the buffers are permanently active.

Example

When pin 19 is at ground potential (low), four of the buffers become active, and any signal at their inputs passes through to their respective outputs.

Operating Voltage

The SN74AC244 has a wide operating voltage range. It can operate from as little as 2 V to as high as 6 V with an absolute maximum of 7 V. On the Gertboard, the Vcc pin ties to the 3.3 V rail. When the IC is operating at 3.3 V (Vcc = 3.3 V), logic 1 is represented by 3.3 V, therefore the GPIO pins of the Raspberry Pi will be safe. When Vcc is 3.3 V an input voltage of at least 2.1 V or higher is required to be recognised as a logical 1, and an input voltage of less than 0.9 V is required to represent a logical 0. The output pins can sink or source up to 12 mA of current.

The SN74AC244 is very popular and becoming difficult to source sometimes however there are many cheaper alternatives that people are not aware of and in the following sections of this article I have made a list of alternative compatible chips. There is also an option for you to build a buffered I/O, which you can use with many Raspberry Pi projects.

This Article Continues...