Replace masked ROM with EPROM, what to do with output enable pin. - Page 1
Forum section: electronics / projects

author: Xenon
author_rank: Contributor
timestamp: October 03, 2021, 04:36:27 pm
content: Currently I am repairing an old Tektronix 4051, and I suspected 1 or more ROMs to be faulty (they are MCM6832 ROMs).The ROM images are available on the internet so I compared them to those with my EPROM programmer. From the 16 ROMs 1 appeared to be faulty. Every read action the checksum was different.Now I am planning to replace the defective ROM with an EPROM, preferrably a 2732 since I have those in stock (a 2716 is an exact replacement). I'll have to make an adapter pcb since they are not directly pin compatible. Testing the ROMs in my programmer was done the same way.The ROM (or the 4051) only has "chip select".The EPROM has "chip select" and "output enable". According to the datasheet the output enable has to go low several microseconds after the chip select.Is this critical? In other words, can I tie the chip select and output enable together so they go low at the same time? Or can I tie the output enable to ground permanently?I can try to find this out by trial and error, but the required knowledge might already be available here on the forum. And there may be more faults in the computer so I cannot test this in a known good system.Thanks in advance.

author: wraper
author_rank: Unknown
timestamp: October 03, 2021, 04:50:46 pm
content: CS of that ROM effectively is the same as output enable in 2732. Tie chip enable permanently and use output enable as CS of MCM6832.

author: Xenon
author_rank: Contributor
timestamp: October 03, 2021, 07:09:01 pm
content: Then I interpreted the following text on the datasheet the wrong way.I'll just start with E to ground. When that doesn't work I'll try it the other way around.The M2732A has two control functions, both ofwhich must be logically satisfied in order to obtaindata at the outputs. Chip Enable (E) is the powercontrol and should be used for device selection.Output Enable (G) is the output control and shouldbe used to gate data to the output pins, inde-pendent of device selection.

author: Benta
author_rank: Super Contributor
timestamp: October 03, 2021, 07:32:14 pm
content: Totally wrong! The /GVpp pin is unimportant and should just be tied low.The /E pin must be connected to the /CE line. Period.

author: wraper
author_rank: Unknown
timestamp: October 03, 2021, 08:01:51 pm
content: Quote from: Benta on October 03, 2021, 07:32:14 pmIt will make time to valid output being twice as slow (you will save some power though), and if you use the slowest grade of 2732 it may become slower than the max delay spec of MCM6832. As you can see, CS actually is just an output enable.

author: Xenon
author_rank: Contributor
timestamp: October 03, 2021, 09:20:16 pm
content: Many opinionsAfter posting my question I found this in another 2732 datasheet:"To most efficiently use these two control lines, it isrecommended that E be decoded and used as theprimary device selecting function, while G shouldbe made a common connection to all devices in thearray and connected to the READ line from thesystem control bus.This ensures that all deselected memory devicesare in their low power standby mode and that theoutput pins are only active when data is requiredfrom a particular memory device."I should have found this earlier, but the discussion is interesting. I'll try out both ways and post my findings here.Know what, I'll also try it with both lines connected to CE.I'm curious.After all it says "to use the control lines most efficiently" and not "to use the control lines correctly". This suggests to me there is more then one way to use them.

author: wraper
author_rank: Unknown
timestamp: October 04, 2021, 07:53:23 am
content: Quote from: Xenon on October 03, 2021, 09:20:16 pmIt will work the same as the variant suggested by Benta. /G input does not matter when the chip is deselected, and when selected it will be always active anyway.

author: gcewing
author_rank: Regular Contributor
timestamp: October 04, 2021, 02:05:12 pm
content: I can't see anything in the datasheet that requires any particular timing relationship between Chip Enable and Output Enable.