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JNZ
September 17th, 2017, 06:25 PM
I need to provide an inverted clock signal to my Versafloppy II board, so I wired together a simple circuit on a prototype board: pin 24 of the S-100 bus (master clock) goes into both inputs of an 74LS38N NAND inverter (pins 1 and 2), and the output (pin 3) goes to bus pin 27. The inverter is powered with a +5VDC regulator and has a 0.1uF tantalum cap across its pins.

The datasheet also lists the maximum time for a transition as 22 ns. If two such transitions are required per clock period, doesn't that mean that it should be able to invert the signal from up to a 23 MHz clock? I'm at just 4 MHz.

However this circuit doesn't seem to output anything more than typical noise at a 240mV DC potential. Anyone know what's wrong with using this chip for inverting a clock? How should it be done?

Chuck(G)
September 17th, 2017, 07:26 PM
The 7438 is an open-collector output. You can either add a pullup to the output or, better, substitute a chip with standard totem-pole output. Unless you're using all 4 sections, try substituting a plain old 7400.

JNZ
September 18th, 2017, 09:52 AM
The 7438 is an open-collector output. You can either add a pullup to the output or, better, substitute a chip with standard totem-pole output. Unless you're using all 4 sections, try substituting a plain old 7400.

Ahh I see. I had picked the part based on availability, so I may try the pull-up resistor method. However, I also have a SN74HCT04 hex inverter on hand. Would that do the job as well?

Chuck(G)
September 18th, 2017, 10:04 AM
I assume that your chip is socketed; an LS00 would be a drop-in substitution (same pinout). It should have enough moxie to drive a couple of cards. If you want the totem-pole variety of the 74LS38, it's the 74LS37, which is pretty uncommon in the wild.

JNZ
September 18th, 2017, 10:31 AM
I assume that your chip is socketed; an LS00 would be a drop-in substitution (same pinout). It should have enough moxie to drive a couple of cards. If you want the totem-pole variety of the 74LS38, it's the 74LS37, which is pretty uncommon in the wild.

Thanks, I actually had a 74LS00N in a half-working N8VEM SBC board I was able to scavenge. It appears to be providing an inverted clock signal now