74LS output protection

krotus · Feb 2, 2011

Hi, I have bus with 2x 74LS374 outputs side and 8x 74LS374 inputs side attached to it. I want to connect mcu (avr) to this bus and add some protection for the case I select some 74LS374 with outputs attached to the bus and forget to switch mcu port to input mode. Resistors in series between mcu and bus would be propable cheapest solution but what value should I use, also with respect to bus (layed on pcb) and 74LS374 inputs capacitance? Thanks.

zuisti · Feb 2, 2011

Yes, 16 resistors will be the solution. Value? As smallest as possible, but it's limited by the AVR and the LS374 output current (source/sink) capacity. Assumed 20 mA there, 220 Ohm will be good (in series between the bus and the outputs of the two LS374 !!).

krotus · Feb 2, 2011

Thanks, but pcb is already etched and assembled so only place where I can put resistors now is between mcu and rest of the bus. Although there is 74LS138 used to control output enable of 74LS374's so there should be always only one of them driving the bus. Maybe except switching? Can I use 74138 to switch directly between 374s, or rather should I switch to some unused output at first, wait there a while and then to another 374?

zuisti · Feb 2, 2011

krotus said:
only place where I can put resistors now is between mcu and rest of the bus.
...there should be always only one of them driving the bus.
In this case that's the right and the only solution : - )
Use 8 pcs 220 Ohm here.
Maybe except switching?
No, do not worry, because the transfer time is very short (< 10 nSec).
Can I use 74138 to switch directly between 374s
Yes

See above : - )

74LS output protection

krotus

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zuisti

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krotus

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zuisti

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