74*240
From BEAM Robotics Wiki
[edit] Octal Buffer / Line Driver with Tri-state Outputs
The '240 is often called "the bicore chip", because we can take advantage of the 240's inverters to turn a single 74*240 into a bicore (in this case, only 2 of the inverters are used, and the rest are used for upping the current so you can drive a motor directly). The '240 also has tri-state outputs, so an enable line can be used to turn its outputs on and off simply (good for adding reversing capability to a 'bot).
Since the '240 gives us "vanilla" (non-Schmitt) inverters, it is usable for either grounded or suspended bicore designs (but better for suspended). See also the 74*04 (which is very similar to the 74*240, but without tri-state outputs, and with about half the output current capabilities), and the 74*244 (which is basically a '240 with non-inverting buffers).
Th '240 can also be used as a motor driver. Commonly the AC variant is used for that purpose.
Note that pins 1 and 19 are the enable (tri-state control) pins. Signals on these pins pass through an inverter before getting to the buffer's tri-state control terminals, which means that a low voltage at 1 & 19 would be a high voltage at the buffer, so the buffer would be enabled. A high voltage on those pins would be a low voltage at buffer's tri-state controls and would turn the buffers off. So tie pins 1 and 19 to ground to enable all of the inverters inside the '240 chip.
[edit] Words of wisdom from Wilf:
74HC240 and similar HC buffers - Vcc=2V-7V but will operate down to 1.0V or less, input voltage trigger / switching level about 1/2 of Vcc, supply current when one input is at switching level = 50 ma (power hungry during slow switching), 30 ohm resistance in series with output with Vcc=5V. Drives small motors. Can give very long bicore time constants if components are carefully matched. Recommended for power smarthead / monocore circuits since time constants remain relatively constant with change in Vcc.
74HCT240 - same as 74HC240 but with "fixed" input switching point of about 1.6V. Faster bicore frequency with same components as HC circuit.
74AC240 and all other AC devices - same as HC but about 10 ohm series output resistance. Three times drive current of HC, draws over 100 ma power supply with input at switch point. Prone to oscillation. No problems / low power when used with fast changing digital input signals and highest efficiency for motor (100 ma) driver applications.
74ACT240 - same as AC but fixed input switching level.
A 74HCT240 Nv net has a shorter pulse duration (given the same resistors and capacitors) than an Nv net built on a 74AC240 or 74HC240.
The 74HC240, 74HCT240, and 74AC240 are all available from long time BEAM supporter Solarbotics
Also see:
- BEAM ICs
- CMOS section of the 7400_Families article.
