Part Number: SN7402
The input(s) is(are) sinking substantial current (milliamperes) when a logical high (+5 VDC / Vcc in this case) is applied to the input. My understanding from decades of practice, and from the specification sheet, is that TTL inputs do NOT sink substantial current when a logical high is applies. The spec sheet appears to indicate that the input current maximum is only 40 µA, maximum, when a logical high is applied.
Either the specifications have changed, or I am having incredibly bad luck with the batches of parts with which I am attempting to work. I will note that I observed this behavior on a prototype circuit board, and replicated it on a breadboard with the SN7402N isolated for test purposes.
Have specifications changed for this part? Have there been instances of manufacturing defects?
Thank you.
Rich
(Richard Bonomo)
ISSUE APPARENTLY "RESOLVED" -- GATE PIN ASSIGNMENT ERROR
I had decided to try to replace the 7402 quad NOR gate 14-pin DIP with a CMOS variety, a CD4001BE, to see if this resolved the issue. I decided to check the pinouts to make sure the two packages were configured the same way. I noticed that the CD4001BE did not have the same pinout as shown on the schematic diagram that I had prepared using Eagle and Eagle libraries. To my shock, I saw that the pinouts on the Eagle schematic, which was based on a "7436" quad NOR-gate (which does not appear in catalogs nowadays) -- e.g. 1 & 2 are inputs, 3 are outputs, did not match the pinouts of the 7402 as per spec sheet -- e.g. 2 & 3 are inputs and 1 is an output, which is like the pinout of the 7400 quad NAND gate. So, all of this time, when I thought I was connecting to a TTL NOR gate input, I was actually connecting to a TTL output!
This explains the odd discrepancy.
This explains the odd discrepancy.
