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I am currently playing around with a 74LS247 BCD to 7 segment decoder. The basic usage for decoding works perfectly fine.

But I would like to test out everything this IC has to offer including the blanking interval for controlling the brightness. According to the datasheet this is done via the BI/RBO pin. But I am confused regarding the way to use this pin.

As far as I understand this it serves as well as an output for cascading the zero blanking to the next segment and as an input for blanking this segment.

But how can I input a logic level into this pin without risking a short circuit? Will I get into problems when I connect it to H or L while it is outputting a different level?

In the datasheet there is a circuit that represents the way this input works, sadly I am not an expert in semiconductor magic to interpret it correctly.

The datasheet can be found here.

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  • \$\begingroup\$ #Matthias, Welcome and nice to meet you. You remind me of my good old Arduino days when I played with logic chips including the BCD to 7 segment decoder. I vaguely remember that it was not that difficult to make it work. Just now I skimmed the datasheet to refresh my memory, and made some reading notes below. (1) The chip is all 5V TTL Logic. (2) I usually put a 330R/470R current limiting resistor in series with any pin that I worry that I might accidently fry it. / to continue, ... \$\endgroup\$
    – tlfong01
    Commented Feb 6, 2021 at 13:38
  • \$\begingroup\$ (3) The notes below the truth/function tables are indeed confusing, especially the "wired AND" comment. I know open collector wired OR pretty well, but never touched any "Wired AND". (5) I usually do the trials and errors by hand, with a jumper wire, BRIEFLY touching any pin to Vcc or ground. (6) Usually if you accidentally short something, it takes a couple of seconds to heat up and melt the plastic. So BEFORE you smell burning plastic, or see white smoke coming out, you can use you finger to touch if any thing hot there. (7) Comments or counter suggestions welcome. Good luck. Cheers. \$\endgroup\$
    – tlfong01
    Commented Feb 6, 2021 at 13:40

1 Answer 1

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There are two ways to look at this:
"White box": interpret the schematics to figure out how exactly any given terminal will behave.
"Black" (opaque) box: just go by table entries.
I recommend starting with the latter:
The ¬BI input currents for a "high" input level are standard.
For a low, it's up to three times that of a standard input.
This does not pose a problem when using ripple blanking: you need to drive the first device in the blanking chain, only.
Wanting a blanking input without ripple blanking, one standard output is guaranteed to be able to drive 4 ¬BI inputs instead of 10, an LS output 7 instead of 20. Short circuit to ground is guaranteed to not exceed 4 mA (standard) and 2 mA (LS); resistors to give a \$V_{in}\$ of ≤0.4 V would be ≤100 Ω and ≤300 Ω, respectively;
"pulling ¬BI low" is the intended way to blank unconditionally (overrides even "lamp test").
One should not try to force ¬RBO high; this is unspecified and gets one into problems more likely than not.

(White box shows two pull-up resistors almost parallel, and an active pull-down.)

(I have no idea what the wire-AND logic in the footnote
¬BI/¬RBO is wire-AND logic serving as blanking input (¬BI) and/or ripple-blanking output (¬RBO). is supposed to mean.)

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