Another clueless question

[toa]

I am doing some preliminary tests on a SN74198 TTL shift register, and I am
getting some weird behaviour. It seems that when I shift the bits in the
register using the clock input, sometimes low bits go inexplicably high. For
example, I start with all bits high, then shift a low bit in from the left.
I then invert the bit input, and continue shifting right, so that the one
low bit is shifted rightward while high bits are being filled in from the
left. Suddenly, as I shift the 4th time, all bits are high. If I shift in 2
low bits initially, one of them goes high at the 4th shift. If I shift in 3
or more low bits, they continue through the register and shift out at the
right as they should.

I tried lowering the input voltage from 5V to 4V. Then everything worked OK.
Thing is, I'm building an ISA bus interface here, so I don't have the option
of lowering the voltage.

I'm using a 7-segment display with decimal point to look at the outputs,
using one segment to show each bit. The display is powered through a
resistor to its common VCC pin, and each GND pin is conneced to a Data Out
pin of the 74198. Yes, terribly amateurish, I know, the luminence of the
segments is inversely proportional to the number of segments lighted and so
on, but it works, and is only makeshift while I experiment. This wouldn't
interfere with the workings of the register, would it?

Any thoughts?

Best regards,
Torgeir Apeland

 

[Mark VB]

I am doing some preliminary tests on a SN74198 TTL shift register, and I am
getting some weird behaviour. It seems that when I shift the bits in the
register using the clock input, sometimes low bits go inexplicably high. For
example, I start with all bits high, then shift a low bit in from the left.
I then invert the bit input, and continue shifting right, so that the one
low bit is shifted rightward while high bits are being filled in from the
left. Suddenly, as I shift the 4th time, all bits are high. If I shift in 2
low bits initially, one of them goes high at the 4th shift. If I shift in 3
or more low bits, they continue through the register and shift out at the
right as they should.

I tried lowering the input voltage from 5V to 4V. Then everything worked OK.
Thing is, I'm building an ISA bus interface here, so I don't have the option
of lowering the voltage.

I'm using a 7-segment display with decimal point to look at the outputs,
using one segment to show each bit. The display is powered through a
resistor to its common VCC pin, and each GND pin is conneced to a Data Out
pin of the 74198. Yes, terribly amateurish, I know, the luminence of the
segments is inversely proportional to the number of segments lighted and so
on, but it works, and is only makeshift while I experiment. This wouldn't
interfere with the workings of the register, would it?

Any thoughts?

Best regards,
Torgeir Apeland

Probably, the load is drawing more current than the output diodes of the
74198 can supply (+/- 20mA). So, the more segments in active state, the
lower the current per output. (because the current is divided over multiple
segments)

To solve it, use one resistor per segment, raise the value of the resistor
you use, or buffer the outputs. Lowering voltage also works, because you
also lower the current, but i wouldn't recommend it.


Mark Van Borm