migry
Member
If this is posted in the wrong place, I am sure that the mods will move it!
I am starting to new project. I want to build a PC-XT circuit based on the 8088 and original Intel chipset parts. Now I recently found a YouTube channel where someone had done just that.
First job it to gather all the parts. BTW I decided to omit the DMA chip, as I don't think it is used in the original XT. Perhaps it is needed for the floppy controller??? So I turned to the usual sources. Since I was looking for old retro ICs, I had to search around. I ended up buying 4 different types of devices from the same seller, one I have used previously. The AMD 8088 looks genuine as does the Mitsubishi 8253. Date codes appear reasonable and printing is ink. The 82C55 look like they have been remarked, and the NOS 8253s have scratch marks on the top (seller says this was likely caused by "dust" in the tube!).
Over the past couple of years I have been burnt by fake re-marked chips sourced either directly or indirectly from China. This I am sure is a well known problem.
So the 82C55 PPI chips. They were sold in twos, supposedly NOS by Harris. I examined them closely when they arrived, and I am suspicious as to whether they are genuine. The markings are laser etched. This is something I have seen on the fake parts (PLCC MC68000) which I had bought, which had been sanded down and then laser etched. I guess this allows slower devices such as CPUs to be marked as if they operated at a faster speed. Who knows what these Chinese fakers are thinking?
For the 82C55s The texture of the plastic on the bottom is not the same as on the top, the bottom is very smooth and shiny. When I check other 40 pin devices in my junk box, I find parts with all sorts of textures, but top and bottom are always the same.
I googled so see if I could find a genuine Harris part. Here is what I found. My gut feeling is that it looks genuine.
Here is the NOS part.
I spoke on the phone to the seller and he swears blindly that it is a genuine NOS and came from a tube marked Harris.
Has anyone any other examples of the Harris parts?
I'm not too familiar with Harris and have no other examples of their ICs.
I have now been trying to test the device. Using a multimeter I have confirmed that the GND and VCC pins are in the correct position. This is done by looking for the input/output clamping diodes. This is strong evidence that these are 8255s as GND and VCC are not in the corners. I have used an Arduino to stimulate the device and it is definitely a 8255. I am able to count on one of the ports using LEDs as a monitor. I simply haven't tried the other ports.
When the ports are input I used a multimeter on the current range to short the input to either GND or VCC. When shorted to GND I was seeing around 400uA of current. Since this is a CMOS part I was expecting more or less zero. Looking at the datasheet I am a bit confused as to what this current ought to be. When shorted to VCC measured current is zero. My concern is that sometimes remarked devices have NMOS devices remarked as CMOS. FWIW I was able to use IPA to remove the MC68HC000-16 marking to reveal the original MC68000-8 markings of a part bought a year or so ago. So either the fakers/re-claimers don't know or care. I am going to try to measure input threshold and Vol/Iol and Voh/Ioh to see if I can tell if it really is CMOS.
It's a lot of effort, and perhaps I am being paranoid, but having been burnt with my 68000 purchases (and other ICs) I want to make sure that the parts are genuine and work.
Thank you for any feedback.
I am starting to new project. I want to build a PC-XT circuit based on the 8088 and original Intel chipset parts. Now I recently found a YouTube channel where someone had done just that.
First job it to gather all the parts. BTW I decided to omit the DMA chip, as I don't think it is used in the original XT. Perhaps it is needed for the floppy controller??? So I turned to the usual sources. Since I was looking for old retro ICs, I had to search around. I ended up buying 4 different types of devices from the same seller, one I have used previously. The AMD 8088 looks genuine as does the Mitsubishi 8253. Date codes appear reasonable and printing is ink. The 82C55 look like they have been remarked, and the NOS 8253s have scratch marks on the top (seller says this was likely caused by "dust" in the tube!).
Over the past couple of years I have been burnt by fake re-marked chips sourced either directly or indirectly from China. This I am sure is a well known problem.
So the 82C55 PPI chips. They were sold in twos, supposedly NOS by Harris. I examined them closely when they arrived, and I am suspicious as to whether they are genuine. The markings are laser etched. This is something I have seen on the fake parts (PLCC MC68000) which I had bought, which had been sanded down and then laser etched. I guess this allows slower devices such as CPUs to be marked as if they operated at a faster speed. Who knows what these Chinese fakers are thinking?
For the 82C55s The texture of the plastic on the bottom is not the same as on the top, the bottom is very smooth and shiny. When I check other 40 pin devices in my junk box, I find parts with all sorts of textures, but top and bottom are always the same.
I googled so see if I could find a genuine Harris part. Here is what I found. My gut feeling is that it looks genuine.
Here is the NOS part.
I spoke on the phone to the seller and he swears blindly that it is a genuine NOS and came from a tube marked Harris.
Has anyone any other examples of the Harris parts?
I'm not too familiar with Harris and have no other examples of their ICs.
I have now been trying to test the device. Using a multimeter I have confirmed that the GND and VCC pins are in the correct position. This is done by looking for the input/output clamping diodes. This is strong evidence that these are 8255s as GND and VCC are not in the corners. I have used an Arduino to stimulate the device and it is definitely a 8255. I am able to count on one of the ports using LEDs as a monitor. I simply haven't tried the other ports.
When the ports are input I used a multimeter on the current range to short the input to either GND or VCC. When shorted to GND I was seeing around 400uA of current. Since this is a CMOS part I was expecting more or less zero. Looking at the datasheet I am a bit confused as to what this current ought to be. When shorted to VCC measured current is zero. My concern is that sometimes remarked devices have NMOS devices remarked as CMOS. FWIW I was able to use IPA to remove the MC68HC000-16 marking to reveal the original MC68000-8 markings of a part bought a year or so ago. So either the fakers/re-claimers don't know or care. I am going to try to measure input threshold and Vol/Iol and Voh/Ioh to see if I can tell if it really is CMOS.
It's a lot of effort, and perhaps I am being paranoid, but having been burnt with my 68000 purchases (and other ICs) I want to make sure that the parts are genuine and work.
Thank you for any feedback.