Gertboard Hack Using SN74AC244 I/O Chip

Here is a Gertboard buffer chip hack. The SN74AC244 buffer chip damages very easily and there may come a time when you cannot source cost-effective replacements. In this hack, I am replacing the original version of the IC with the SN74AC241N, which is very readily available. If you happen to have this IC, then this hack might be useful. This is because this IC is a tri-state non-inverting octal buffer chip belonging to the 74AC family, the same as the SN74AC244 used on the Gertboard. This means that the chip will operate on a 3.3 V supply and the buffer input/output will operate at CMOS levels.

Complementary Control Lines

The diagram above shows the pinout of the SN74AC241N octal buffer IC. This chip is identical to the SN74AC244 in every way except control pin 19 is active-high, whilst on the SN74AC244 pin 19 is active-low. As you can see from the diagram above, pin 1 is active-low whilst pin 19 is active-high, in a complementary arrangement. When pin 1 is logic 0, the internal buffers become active. However, pin 19 requires logic 1 state to enable the buffers. A complementary arrangement is very useful in a bi-directional bus because when data is travelling in one direction one set of buffers are enabled and another set disabled, by a single control signal.

Hack Wiring Method 1

On the Gertboard, pin 1 and pin 19 connect to ground thereby keeping the buffers permanently enabled. This IC will work on the Gertboard if somehow pin 19 connected to Vcc instead. One solution would be to cut the track, which would void the warranty. However, I seem to remember a trick from the old days, which is to bend the chip pin 19 so that it was straight as shown above in the diagram. This way it no longer plugs into the chip socket. Once isolated in this way, it connects to Vcc by soldering a small length of jumper wire to the Vcc pin, which is next to it. The great thing is that your board also remains compatible for a SN74AC244 chip as well.

Hack Wiring Method 2

If your soldering is good then it could connect like this. Each chip pops out independently if required. There are less trailing wires and you will not be overloading the U3 chips +Vcc PCB trace.

Warning

Please note that any kind of soldering directly on the printed circuit board will void your guarantee. Therefore, do not solder on the board if you wish to keep it intact. In the following sections of this article, I have compiled a list of chips currently available that are compatible with the SN74AC244 to avoid having to do this hack; however, stocks of this chip are becoming difficult to source.

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