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

 

hi keii.
The resistors are shown on the MCU datasheet
E

 

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.

 

hi keii.
The resistors are shown on the MCU datasheet
E
View attachment 290226

Thanks.

 

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.

 

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