Can you explain the logic function of this TTL gate? I think it is $$ Y = \overline{AB+CD} \ $$ but I am not sure. If $$ V_1=5V \\ V_2=0V $$ does the transistor Q1 work in reverse-active mode?
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2\$\begingroup\$ Why not use your favorite simulator and see for yourself? \$\endgroup\$– TylerCommented Apr 13, 2017 at 14:04
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2\$\begingroup\$ ahhhhhhhhh the AND_OR_expander logic...........these used to be available, with external pins (two) across Q3 for arbitrary expansion \$\endgroup\$– analogsystemsrfCommented Apr 13, 2017 at 14:13
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1\$\begingroup\$ @Tyler, that's a dreadful suggestion - skive off to the simulator when you can simulate it in your head in 2 mins! Where's the fun in switching your mind off. (Anyway, give me half hour, I'll have an answer...might be longer :-) ) \$\endgroup\$– TonyMCommented Apr 13, 2017 at 14:21
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\$\begingroup\$ @analogsystemsrf you're one of the very small number of engineers who remembers them. There were other, similar, ICs that TI apparently made for a single customer then tried to sell to more customers. I was annoyed that they took up space in the TTL data book I had to thumb past to find what I was looking for. \$\endgroup\$– user128351Commented May 15, 2017 at 1:41
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\$\begingroup\$ Q1,Q2,Q7,Q8 are the kind of inputs all the 54/74 ICs had, including 54L/74L, 54H/74H, 54S/74S. 54LS/74LS had diode inputs to get AND function, rather than transistors. \$\endgroup\$– user128351Commented May 15, 2017 at 1:46
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does the transistor Q1 work in reverse-active mode?
Yes, but the current gain is very low so it will not be able to override a logic Low on the other input (when Q2 is pulling down).
answered Apr 13, 2017 at 17:51