I'm doing a bit of testing with the SN74LS245 IC which is an octal bus transceiver. According to what I understand in the datasheet and the given truth table, pulling the DIR (pin 0) high (5V) and the OE (pin 19) low (0V) should result in the A side channels to be passed to the B side channels. (Ax to Bx)

I have a breadboard wired up in this configuration:


However, instead of measuring a 5V potential at B2 when 5V is present at A2, no voltage is passed through the other side. I've tried fiddling around with the direction pin but receive the same results in either direction.

Am I missing something in the datasheet or is it a possibility I have come across a bad chip?

  • \$\begingroup\$ You are correct. Try another A & B. \$\endgroup\$ Commented Aug 31, 2019 at 2:11
  • 2
    \$\begingroup\$ You should not connect the 5V to the I/O pins directly. If you make a mistake with DIR your I/O is shorted. Use a 1K series resistor. Your test will work just as well but your IC might live longer. \$\endgroup\$
    – Oldfart
    Commented Aug 31, 2019 at 4:15
  • \$\begingroup\$ The "nominal pull-up resistor" for the LS TTL family is \$18\:\text{k}\Omega\$. See the Texas Instruments', "The TTL Data Book for Design Engineers." for that reference. The worst case high level input current is \$20\:\mu\text{A}\$. (However, some LS TTL ICs would use an internal \$2\:\text{k}\Omega\$ pull-up resistor.) \$\endgroup\$
    – jonk
    Commented Aug 31, 2019 at 7:17
  • \$\begingroup\$ @jonk There are no pullups on the LS245. It would be impossible to 3-state the outputs if it had them. \$\endgroup\$
    – Finbarr
    Commented Aug 31, 2019 at 8:06

1 Answer 1


It sounds like you may have a short on the output or the chip is damaged. The wiring looks proper.


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