Pixie thermometer question

[Jonathan Peakall]

Hi All,

I am finally getting around to soldering up a pixie tube thermometer
that I bread boarded quite a while back. It uses a PIC and two
SN7414N's. On the bread board I have 1k resistors in series from the
output pins of the PIC to the input pins of the SN74141Ns.  Usually I am
using shift registers but with a two digit thermometer I am coming
directly off of the PIC pins. I can't seem to remember why I put the
series resistors in there. Are they required?

Forgetfully yours,

Jonathan

[threeneurons]

I'd get rid of the 1Ks, and go directly. They are powered, from the same 5V supply ? Unterminated TTL inputs float to ~1.6V, look like an "iffy" logic-1. To drag them down to solid logic-0 (0.8V min) requires a "sink" current of 1.6mA for old standard TTL. A 1K is too big. Unused TTL inputs are almost always terminated high, as the "source" current to a good logic-1 (>2.0V) is much smaller than the sink current. To terminate original TTL low, would require a resistor ~220 ohms.

[Jonathan Peakall]

Thanks Three! I think I was worried about the HV being just one chip away from the MCU. I'm not an EE and have never even played one on TV. I'll go direct, it's easier anyway.

Jonathan

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