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I'm driving a large number of parallel UART inputs from a single UART output, at 2 Mbit. Currently, I'm driving with a Atmega32u4 using a 70 Ohm current limiting resistor (which in theory would allow exceeding the specified maximum output current of 40 mA, but in practice doesn't do that for enough time to hurt the device.)

I need this driver to be able to tri-state or disable, because I also read data from the other devices on this TTL-level UART bus.

I am seeing some signal quality issues, though. I'd like to clean up the signal by using a lower-impedance driver -- really, a single channel of buffer with tristate ability and high bandwidth would be great! (I can accept 50 ns rise/fall times into 1000 pF, but no worse than that.)

My first idea was to use a regular half-bridge driver, like the L293E. However, that doesn't have fast enough rise/fall times for this signal.

My second idea was to search DigiKey parametric search. I came up with a number of possible devices, but none of them actually has all of the features I need. Either they are missing a tristate/disable input, or they have too slow rise/fall times. The only device I've found with both fast rise/fall and tri-state/disable inputs is the EL7232. Unfortunately, this device is inverting! I can't find a mention of a non-inverting version of this device in the data sheet, or through applying DigiKey search.

It would seem like there should be a chip that does what I need it to do, but I can't find it. Does this ring a bell for anyone? Am I missing something? Is my best bet to put an inverter (N-channel low-end switch plus pull-up resistor) in front of the EL7232, consuming more precious board space?

Edit: I keep forgetting to add this requirement when I ask questions: I am a dinosaur who does not know how to operate a toaster oven, and also have a large existing stash of through-hole parts I'd like to not have to throw away. I am only interested in through-hole solutions.

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Look into the TSC427 part. The Maxim equivalent would be the MAX627. These parts work over a Vdd range of 4.5 V to 18 volts and can drive 1000 pF loads with rise times typically at 20 nsec. Digikey shows stock under both part numbers in several package styles.

WHen you use these parts make sure to provide good bypass from Vdd to GND right by the package. Sometimes is is necessary to add a small value series resistor at the output to eliminate ringing of the output waveform.

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  • \$\begingroup\$ I looked at those, and they don't have a disable input, so I can't use it on a bidirectional multi-drop TTL UART bus as described in the problem statement :-( Well, at least not without some other way of driving it into "disable" or "tristate." What's extra annoying is that there are two "nc" pins on that chip, so it totally could have a disable... but it doesn't. \$\endgroup\$ – Jon Watte Apr 6 '13 at 6:58
  • \$\begingroup\$ The closest I can come is the EL7155, which is non-inverting and has great drive capability, but is only available is SOIC, not DIP. And $6 each (eep!) intersil.com/content/dam/Intersil/documents/fn72/fn7279.pdf \$\endgroup\$ – Jon Watte Apr 30 '13 at 22:07
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    \$\begingroup\$ The manfacturer data sheet shows parts in DIP package (EL7155CN). You may want to contact them to see if samples are available in DIP and then when you go to PCB you could easily accomodate SOIC-8 at that time. \$\endgroup\$ – Michael Karas May 1 '13 at 10:11

protected by W5VO Apr 5 '13 at 2:32

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