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).
The '240 can also be used as a motor drive. 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 signal at 1 & 19 would be a high signal at the buffer, so the buffer would be enabled. A high signal on those pins would be a low low 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 switching 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.
[edit] Datasheets
