# SN74LS181: The comparison function, A=B, not operating

I have six SN74LS181N-B circuits for arithmetic functions. I intend to utilise the A=B output, which is the comparison function. Sources have indicated that this function operates, if subtraction is acted, and the output word, (F), is 1111.

However, this does not seem to occur; I tried both situations of equality and inequality to observe if the pin is inverted; I even tried when the whole output word, (F), is 1111.

Additionally, I found that the Carry pins, (Cn, Cn+4), are inverted and thus suspect the output and input words are inverted. However, if I use normal averted inputs, it operates proficiently. Do I have to utilise external inverters?

• I should have stated that the pin is 0, (low), during the both situations. – C. Caireallain Feb 6 '18 at 1:50
• Check all ins and outputs vs the datasheet – Tony Stewart Sunnyskyguy EE75 Feb 6 '18 at 2:26
• I am not sure what thou are enquiring? If you mean the operation inputs, (S0-3), well, I am certain that they were 0110, which is the subtraction operation and the carry input, (Cn), was correct. If thou meanest a total datasheet of every input and output case, there is not one for the 74LS181. However, there is a website, which allows an interactive simulation of the circuit, but this predicts what I am attempting to achieve with my chips; for example, the A=B pin is high, if the two inputs are equal. – C. Caireallain Feb 6 '18 at 2:38
• I mean follow the schematic from any output to all the connected inputs and verify each voltage logic level . Start with power and gnd. jameco.com/Jameco/Products/ProdDS/46973.pdf – Tony Stewart Sunnyskyguy EE75 Feb 6 '18 at 2:45

It's difficult to be sure that we're interpreting everything correctly without you supplying a schematic diagram (a picture is worth a thousand words etc.).

However your main question has a clear explanation:

I intend to utilise the A=B output [...]

From the datasheet:

• "The A = B output is open-collector so that it can be wire-AND connected to give a comparison for more than four bits."

See this extract from the datasheet - the A = B output (which I've marked in red) is open-collector, whereas the other outputs are standard "totem pole":

Therefore you can't measure a meaningful change of state between low & high, unless a suitable pull-up resistor is added between the A = B output and VCC.

I think you are saying that the A = B output always remains low. That is expected. An open-collector output pin without a pull-up will always appear to be low (either it's actively driven low or it is undriven, which will also measure as low with a multimeter - read about open-collector outputs for more details)

Try a 4.7k pull-up to VCC on the A = B pin and re-test. You should find that the A = B output is then low, unless you correctly configure the chip and supply equal input words, when it should become high to indicate equality.

• I thank thee. I found that 470 ohms achieves a sufficient voltage. I blame myself that I did not confer to the information of the output circuits. – C. Caireallain Feb 8 '18 at 23:41