The 4021 and 74LS165 are <i>similar</i>, inasmuch as they&#x27;re both PISO shift registers, but they&#x27;re not the same part. Here&#x27;s representative datasheets for both parts from TI:<p><a href="http:&#x2F;&#x2F;www.ti.com&#x2F;lit&#x2F;ds&#x2F;symlink&#x2F;cd4021b.pdf" rel="nofollow">http:&#x2F;&#x2F;www.ti.com&#x2F;lit&#x2F;ds&#x2F;symlink&#x2F;cd4021b.pdf</a><p><a href="http:&#x2F;&#x2F;www.ti.com&#x2F;lit&#x2F;ds&#x2F;symlink&#x2F;sn74ls165a.pdf" rel="nofollow">http:&#x2F;&#x2F;www.ti.com&#x2F;lit&#x2F;ds&#x2F;symlink&#x2F;sn74ls165a.pdf</a><p>I don&#x27;t think the differences are relevant in this application, though.