3 Dec
2006
3 Dec
'06
1:02 p.m.
I know the "right" answer to this one--you drive 220/330 ohm
terminated loads with 7438-type OC drivers. But, going a bit afield
from "correct", can one drive a single
220/330 ohm terminated load at
the end of a 3m length of ribbon cable at 1 MHz
using plain old totem-
pole STTL output? How about an LSTTL or HCT totem-pole driver?
You can assume that the receiver is a 74LS14.
Thanks,
Chuck
3 Dec
3 Dec
1:44 p.m.
the normal totem pole output for S TTL logic will only sink 20 ma to
guarantee a logic low. The 220 ohm resistor to +5 will be 22.7ma, so
you are exceeding the chip's capability by some amount. LS and HCT
can sink even less current.
best regards, Steve Thatcher
At 01:02 PM 12/3/2006, you wrote:
I know the "right" answer to this one--you
drive 220/330 ohm
terminated loads with 7438-type OC drivers. But, going a bit afield
from "correct", can one drive a single 220/330 ohm terminated load at
the end of a 3m length of ribbon cable at 1 MHz using plain old totem-
pole STTL output? How about an LSTTL or HCT totem-pole driver?
You can assume that the receiver is a 74LS14.
Thanks,
Chuck
2:09 p.m.
On 3 Dec 2006 at 13:44, Steve Thatcher wrote:
the normal totem pole output for S TTL logic will only
sink 20 ma to
guarantee a logic low. The 220 ohm resistor to +5 will be 22.7ma, so
you are exceeding the chip's capability by some amount. LS and HCT
can sink even less current.
I was thinking of driving the lines with an LS540. which has an I(OL)
of 24 ma per output. What I was curious about was if the low-level
sink current is sufficient, will I run into problems with the totem-
pole pullup? In other words, do I really need OC drivers?
Thanks,
Chuck
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