When to use Pull-Up Resistors on read/write pins of Memory connected to the 8085

Thread Starter

keiichicom

Joined Apr 26, 2022
76
Hi,

I noticed that 4.7k ohm pull-up resistors are used in this circuit on the read/write memory pins: Saundby's MAG 85(scroll down to yellow text), but in other circuits where the read/write / io/m signals are decoded by the 74138 for example, pull-ups are not used? Is it because in the MAG 85 circuit, the signals are connected directly to the memory device?

Thanks,
Kei
 

Papabravo

Joined Feb 24, 2006
21,262
I believe it is related to the possibility of having DMA devices in an 8085 system. During a period of time when neither the processor or a DMA device has positive control of the bus, those pullup resistors ensure that those control signals have no opportunity to float. Regardless of whether DMA devices are present or not, they cannot possibly hurt.
 

Thread Starter

keiichicom

Joined Apr 26, 2022
76
I believe it is related to the possibility of having DMA devices in an 8085 system. During a period of time when neither the processor or a DMA device has positive control of the bus, those pullup resistors ensure that those control signals have no opportunity to float. Regardless of whether DMA devices are present or not, they cannot possibly hurt.
Thanks, makes sense.
 

nsaspook

Joined Aug 27, 2009
13,434
Yes, it was very common for those control signals to be OR/TRI-State connected so other signals could take over the memory bus, pull-ups were common. One of my early 8080 video controllers allowed the display TTL/CMOS video memory logic to run the memory bus during video active and then switch to the local 8080 for updates to display memory during overscan and blanking periods. A second 8080 board ran applications programs that sent data to the video display

https://forum.allaboutcircuits.com/threads/mixing-74-ls-hct-with-diode-logic.163109/post-1432058
 
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