Torben
Well-Known Member
Hi all,
I'm having an interesting problem with the outputs of the 74LS90 chip. For quick background, I'm making a game scorer using 2 identical 74LS90/74LS47 circuits. Works great--except:
I have the main circuit on a central PCB, and want to mount the 7-seg displays remotely from the main board. I have 6-pin headers on the main board and 6-conductor ribbon cable running from there to the display board, where the 74LS74 decoder, current limit resistors, and 7seg display are.
Now this all works just dandy...*iff* my ribbon cable is less than about 60cm long. Any longer, and the 74LS90 starts counting funny...in fact, it will reliably jump straight from 3 to 12 (BCD, that is; when triggered while at 3, the outputs of the 74LS90 will go from HHLL to LLHH instead of going from HHLL to LLLH as it should).
Doesn't matter if there is anything on the far end of the ribbon cable, by the way. Just plugging a too-long ribbon cable into the header will cause the outputs of the 74LS90 to behave as described.
I have tried this with and without a 100nF cap on the power pins of the 74LS90 and nothing changed. However, I have a Mylar cap there now (I think--it's about 1cm x 1cm x ~0.3cm, and looks like a yellow core of
some kind inside a thin clear coating). I'm going to try different kinds
(i.e. ceramic, electrolytic) to see if that helps.
So finally my questions:
1) Does anybody have any links to information on how long a line you
can drive from the outputs of a 74LS90?
2) Would you typically want to drive the longer lines using transistor buffers or something (I gather there are IC buffers with several built in)?
3) Any other hints/links to good reading? I've googled the heck out of this one.
Thanks all who read this!
Torben
I'm having an interesting problem with the outputs of the 74LS90 chip. For quick background, I'm making a game scorer using 2 identical 74LS90/74LS47 circuits. Works great--except:
I have the main circuit on a central PCB, and want to mount the 7-seg displays remotely from the main board. I have 6-pin headers on the main board and 6-conductor ribbon cable running from there to the display board, where the 74LS74 decoder, current limit resistors, and 7seg display are.
Now this all works just dandy...*iff* my ribbon cable is less than about 60cm long. Any longer, and the 74LS90 starts counting funny...in fact, it will reliably jump straight from 3 to 12 (BCD, that is; when triggered while at 3, the outputs of the 74LS90 will go from HHLL to LLHH instead of going from HHLL to LLLH as it should).
Doesn't matter if there is anything on the far end of the ribbon cable, by the way. Just plugging a too-long ribbon cable into the header will cause the outputs of the 74LS90 to behave as described.
I have tried this with and without a 100nF cap on the power pins of the 74LS90 and nothing changed. However, I have a Mylar cap there now (I think--it's about 1cm x 1cm x ~0.3cm, and looks like a yellow core of
some kind inside a thin clear coating). I'm going to try different kinds
(i.e. ceramic, electrolytic) to see if that helps.
So finally my questions:
1) Does anybody have any links to information on how long a line you
can drive from the outputs of a 74LS90?
2) Would you typically want to drive the longer lines using transistor buffers or something (I gather there are IC buffers with several built in)?
3) Any other hints/links to good reading? I've googled the heck out of this one.
Thanks all who read this!
Torben