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I am poor at understanding what datasheet symbols denote. Please help me decode what might be the drive current of receiver logic pin.

Here is the link to datasheet: https://www.ti.com/lit/ds/symlink/sn75176a.pdf

Receiver pin being pin number 1

In the datasheet "6.6 Electrical Characteristics – Receiver" mentions: IOS Short-circuit output current min –15 and max –85 mA Is this what I am looking for?

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2 Answers 2

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It is specified for driving LSTTL loads (much more sink current than source current). So guaranteed output levels when sourcing 400uA or sinking 8mA.

It is not guaranteed to drive HC inputs, where you might need 0.7Vcc to guarantee a high (plus some noise margin), so you might need a pullup resistor or to use an HCT input. 2.7V is guaranteed, which is not enough.

There is also an error in the datasheet, it should say \$I_{OL}\$ not \$I_{OH}\$ in the \$V_{OL}\$ specification.

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  • \$\begingroup\$ But should'nt the values -400 micro amps and 8 mA as mentioned under TEST CONDITIONS signify that what the voltages were observed to be under those test contions and NOT signify that they are maximum source and sink currents, as they may change under different testing conditions. \$\endgroup\$
    – Bubu
    Commented Jul 15, 2022 at 20:28
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    \$\begingroup\$ They are the test currents for which the output is guaranteed. If you load it more heavily the voltage drop could be higher than guaranteed. If you load it more lightly, it might be less. There is a number (-15mA min/-85mA max) for high output shorted to ground. There is no number for shorted to Vcc (probably it is not safe to do so - the chip could overheat and be damaged). Neither number should be important since the receiver is designed to drive logic inputs, not other things. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jul 15, 2022 at 20:47
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This is an ancient TTL device (not just TTL-compatible!). It has a curve similar to any other part in that family:

I_OH curve

From: https://www.nutsvolts.com/magazine/article/understanding_digital_logic_ics_part_2

And further, check the TTL family handbook e.g.:
http://ecelabs.njit.edu/fed101/resources/MotorolaTTL_Manual.pdf

The point where the curve intercepts the mA axis, is where the short-circuit current spec is measured. Presumably, the mostly-flat slope of this curve varies from part to part, hence the rather wide spread in this parameter. This is not a very useful value for practical purposes, as zero voltage means zero output power; keeping under 10 or 5mA high-output current is a reasonable plan.

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    \$\begingroup\$ Thank you, this seems very resourcesful and will be much helpful in my designs \$\endgroup\$
    – Bubu
    Commented Jul 16, 2022 at 15:52

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