I'm making a circuit to program the AT89S52 microcontroller in a serial fashion as follows:


Call me paranoid but this time I'm using resistors on all outputs to the micro as well as RC networks on the reset and clock lines. I also am using a jumper on the EA/VPP line in case I need to use it for programming as well.

I am using NOR gates (I know I know, I'm using the european version instead of the US version of the symbols but they're designed more neatly) as a latch to control all communications.

Because this design (that I made the PCB for) calls for 74HC series of chips, I happen to own a 74HCT02 NOR gate, but I don't own any 74HC02 NOR gates.

This circuit is powered with regulated 5VDC and the parallel port is set to run in Standard Parallel port mode with no bidirectional capability.

If I replaced the 74HC02 with 74HCT02, would the circuit still work without the need for any extra parts?

  • \$\begingroup\$ If the differences between the 74HC02 and 74HCT02 are not sufficient, then I don't see why it would stop you. \$\endgroup\$
    – user103380
    Commented Aug 2, 2018 at 17:36
  • 1
    \$\begingroup\$ PC parallel port standard specifies TTL logic levels, so your circuit should be using an HCT part. Some modern PCs only put out 3.3V, which may not be enough to properly drive 5V CMOS logic. \$\endgroup\$ Commented Aug 3, 2018 at 3:25

2 Answers 2


The outputs are the same and both input thresholds are compatible with minimal noise <1Vpp. 74HCTxxx Vth=1.5V +- x % and 74HCxxx Vth =Vdd/2 +- xx % over temp.


I'm having similar problems with other config and just to point out, if you read the Phillips datasheet, which by the way is wonderful reading, the 74HC has some more variations which are dependent on the Vcc input. HCT are more stable and have fewer variations, also they have some internal plus:

The 74HCT input stage is similar to that of a 74HC device. It has the same characteristics for LSTTL levels as a 74HC input has for CMOS levels, so there is no trade-off in speed or power dissipation. The switching threshold is lower, 1.4 V at VCC = 5 V. In addition, the 74HCT input circuit, shown in Fig.24, has an enlarged n-channel transistor (N1) and a level-shift diode (D3) has been added. The natural drain voltage of the p-channel transistor (P1) is approximately VCC − 0.6 V, but when the input voltage is LOW, an auxiliary pull-up transistor (P2) raises this to VCC, cutting off p-channel transistor P3 completely. The input stage is well matched to the load presented by the second stage so that symmetrical propagation delays are obtained.

So HCT have some enhancements and the price difference is irrelevant, I think HCT would work better.


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