help IDing an 1986 vintage ROM chip - Page 1
Forum section: electronics / projects

author: gardner
author_rank: Regular Contributor
timestamp: October 19, 2017, 04:07:01 pm
content: Hi all. Over athttps://www.eevblog.com/forum/vintage-computing/repairing-a-vt220/I am hacking on an old DEC VT220 to make it do something it wasn't meant to.At the moment, I am trying to ID the ROM in the main board. It's a mask programmed one, custom to DEC, but it looks to be a Signetics CM83815N. I can't see that this is a generic type. I'd like to read it with my TL866 and see about bodging a replacement EPROM in with some slight alterations.Is there any chance this is not a 27C256 JDEC compatible ROM?Any help identifying the actual ROM specs would be greatly appreciated. Thanks.

author: Kjelt
author_rank: Super Contributor
timestamp: October 19, 2017, 04:46:39 pm
content: Check identify the pins on the ic by tracing them back to the microcontroller/processor.Publish the results here. Good chance the pinlayout is 27xx(x) compatible but have to proof it first before accidentally destroying the rom.Then read it out at multiple voltages 4V, 4,5V and 5V and check if the three files are identical. With this you can sometimes retrieve the bits that are corrupt or have a charge that is at the 0/1 boundary.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 19, 2017, 05:21:08 pm
content: Reasonably certain that's a Signetics logo. "CN83815N" is indeed the part number (N suffix meant PDIP package, etc.). With this to get you started, a little digging on Google might be able to scare up a datasheet. Unless the part number was a custom SKU for DEC, entirely possible back in the 80's.Signetics was purchased by Phillips, so there may be something down that road too.Happy sleuthing!EDIT: I see that 83815 is also an old Ethernet controller from National (now TI). Generally IC manufacturers try to avoid using a part number that another manufacturer has already assigned to something, to avoid confusion and also in case they decide to second-source the original (in which case they'd have two different parts with the same number). Any chance this is a network interface chip and not a ROM?

author: Gyro
author_rank: Super Contributor
timestamp: October 19, 2017, 05:29:23 pm
content: From what I remember of the DEC programmable part number allocation system, to complete 23-xxxxx part number allocation it was necessary to submit both hex files and verification master parts to the coordinator. The hex files were sent out to the part manufacturer and the master parts (for an 8-bit device like that they would almost certainly have been 27xxx EPROMs) used to physically compare against test samples of the mask parts.Your chances are good that it will read as a 27xxx EPROM (with appropriate caution).Edit: Just a note for anyone else who was around at the same time. I just did some searching and Reggie Schreiner (the prog. parts coordinator more or less for ever) sadly died in 2007.

author: edavid
author_rank: Super Contributor
timestamp: October 19, 2017, 05:40:05 pm
content: Quote from: gardner on October 19, 2017, 04:07:01 pmYes, there is definitely a chance, although that is the most likely. You should scope out pin 1 to make sure it's not a chip select.

author: duak
author_rank: Super Contributor
timestamp: October 20, 2017, 12:25:57 am
content: Here's a link to a site with info and manuals on the VT220:http://manx-docs.org/details.php/1,2961I believe it's an 8KB or 16KB device.I really liked using those terminals - the smooth scroll was so elegant that I implemented it in a product's video display even though it slowed things down.Cheers,

author: amyk
author_rank: Super Contributor
timestamp: October 20, 2017, 12:42:05 am
content: According tohttps://archive.org/details/bitsavers_decterminacAug83_1797611there is one 23128 and one 2364 ROM.

author: gardner
author_rank: Regular Contributor
timestamp: October 20, 2017, 03:44:09 am
content: Quote from: duak on October 20, 2017, 12:25:57 amThanks, but I have that one already. The back story is mostly on the other thread I mentioned.The board I have is almost three years newer than that manual and is greatly revised compared to the 1984 rev. Mine has a single ROM, not two. It also has a lot of glue logic in what was likely a pretty early GAL in a large QFP. As the original had altogether 24K in two ROMs, I'm guessing mine is a 32K single-chip.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: October 21, 2017, 04:18:07 am
content: Its definitely a Signetics part. It may be a one-time programmable ROM so possibly a EPROM with no erasure window, maybe a character generator chip. If that's the case it should have a standard EPROM pinout.

author: gardner
author_rank: Regular Contributor
timestamp: October 28, 2017, 09:45:48 pm
content: After scoping the pins and confirming that the signals are plausibly like a 27C256, I went ahead and read it with my TL866 set to a Philips 27C256 part. It says it read fine and with the ROM back in place the VT220 is running with no ill effects.I now need to get stuck in with some 8051 disassembly tools.Thanks all.