Testing Galaksija's memory

26.09.2017 20:13

Before attempting to restore the damaged laminate of Mr Ivetić' Galaksija I wanted to have some more confidence that the major components are still in working order. The fact that the NAND gate in the character generator patch was still working correctly gave me hope that the board was not connected to a wrong power supply. That could do serious damage to the semiconductors. Still, I wanted to test some of the bigger integrated circuits. Memory chips are relatively straightforward to check. They are also mounted on sockets on this board, so they were easy to remove and test on a breadboard.

Character generator ROM on the Galaksija circuit board.

This is another post in the series about restoration of an original Galaksija microcomputer. Galaksija is a small home microcomputer from former Yugoslavia that was built around the Z80 microprocessor. It used EPROMs, a predecessor to modern flash memory, to store its simple operating system. As it was common at the time, Galaksija could not update its system software by itself. In fact, western home computers typically stored such software in mask ROMs. There, code and data was programmed by physically etching a pattern into the metal layer of the chip. Even though Yugoslavia had semiconductor industry that was capable of making ROMs, producing a custom chip was not economical for Galaksija, which had relatively low production numbers.

EPROMs removed from the Galaksija circuit board.

Galaksija originally came with two EPROMs. The first one, called ROM A in the manual and marked Master EPROM here, contains 4 kB of Z80 CPU machine code and data for basic operations. It includes specialized functions related to the hardware: video driver for generation of the video signal, keyboard read-out as well as modulation and demodulation routines for saving data to an audio cassette. Some higher-level functions are also included. There's a simplistic terminal emulation with a command-line interface, a stack-based floating point calculator and a BASIC interpreter based on the TRS-80. My incomplete Galaksija disassembly contains more details.

The second EPROM is the character generator ROM. Galaksija's video output is designed fundamentally around text. The frame buffer contains only references to characters that are to be drawn on the screen. How these characters look, the actual pixels you see, are stored in the character ROM. This is similar to how text mode worked on old PCs and was done to limit the RAM use. In fact, a bitmapped image of the whole screen would not fit into the 2 kB of Galaksija's RAM. Of course, this means that only very limited graphics can be displayed. By sacrificing a lot of RAM and hacking the video driver, some limitations can be worked around.

Iskra EMS6116 static RAM on a Galaksija computer.

Galaksija uses static RAM for its working memory. Using costly static RAM was obsolete even in the early 1980s and is the main reason why Galaksija originally only had 2 kB of RAM (which could be upgraded to 6 kB by inserting up to two more identical 2 kB chips). The much larger dynamic RAM would require more complicated circuitry to interface with the CPU and was only added in the later Galaksija "Plus" upgrade. Interestingly, the Z80 CPU was originally meant to use dynamic RAM and includes functionality to perform the required refresh cycles. However in Galaksija this function was instead used for video generation. This board uses a rare EMS6116 RAM chip made by Iskra Semiconductors.

Galaksija's ROM A connected to Arduino Mega.

After carefully removing all three memory chips from the board I wired them up to an Arduino Mega using a breadboard and a rat's nest of jumper wires. I used a slightly modified Oddbloke's RomReader sketch for dumping the EPROM contents. Since the 6116 RAM has an electrical interface that is very similar to 27-series EPROMs I also used this sketch as a base of my RAM test. The RAM test sketch first wrote a test pattern (bytes 00, FF, AA and 55) to all RAM addresses and then read it out to check for any bad bits. Sources for both Arduino sketches are available here.

The first few runs of the EPROM dumper showed that ROM A didn't read out correctly. Its contents differed from what I had on record and consecutive reads yielded somewhat different results. After double-checking my setup however it turned out that my Arduino Mega board only puts out around 4.5 V on the +5 V supply. This is on the lower specified limit for these EPROMs, so it could explain occasional bad bits. After supplying a more stable voltage to the EPROM from a lab power supply, ROM A read correctly. Its contents were exactly the same as what I had on record (and what I use on my Galaksija replica).

Two variants of the Galaksija character set

Similarly, the RAM and the other EPROM also checked out fine. However, in contrast to ROM A, the character ROM contents differed from what I had expected. After a closer look at the binary dump (using chargendump tool from my Galaksija tools to visualize its contents) it turned out that the difference is in characters 0 and 39 (ASCII hex codes 40 and 27 respectively). These two characters are used to draw the two halves of the logo that is displayed before the distinctive Galaksija READY command prompt.

Galaksija screenshot

The character ROM I use on my replica contains an arrow-like logo of Elektronika inženjering. The ROM in this Galaksija's image contains the game-of-life glider logo of Mipro design. Both of these logos are etched into the copper on the solder side of the Galaksija circuit board:

"design mipro" logo on Galaksija PCB.

Elektronika inženjering logo on Galaksija PCB.

I don't know why there are two versions of the character ROM in existence and how old each of them is. As far as I know, both of these companies were involved in the manufacture of the original do-it-yourself kit parts (including the PCB and the keyboard). Wikipedia currently says that later factory-built computers were built by Elektronika inženjering, so it is possible that the arrow logo version is the more recent one. The blurry screenshot from the original Galaksija manual suggests that the glider logo was used when the screenshot was made. This seems to confirm that the glider logo is older.

Figure showing Galaksija's character set from the Galaksija manual.

In any case, both versions of the ROM seem to already float around the web, so this discovery isn't terribly exciting. The Galaksija Emulator for instance comes with the glider logo version. As far as I can remember, I originally obtained my arrow logo ROM images from the Wikipedia page. The article used to contain hex dumps, but they were since then deleted due to copyright and non-encyclopedic content concerns.

In conclusion, everything worked as expected, which is great news as far as the restoration of this Galaksija is concerned and a green light to proceed to fixing the PCB. It's also a testament to the reliability of old integrated circuits. I was pretty sure at least the EPROMs have discharged. The datasheet mentions that normal office fluorescent lighting will discharge an unprotected die in around 3 years. Considering that the chips were most likely programmed more than 30 years ago, it is surprising that the content lasted this long (the bit errors at low supply voltage I've seen might be the first sign of the deterioration though). It's also surprising that the Iskra EMS6116 survived and passed all the tests I could throw at it. Domestic chips did not have the best of reputations as far as reliability was concerned, but at least this specimen seemed to survive the test of time just fine.

Posted by Tomaž | Categories: Digital

Add a new comment

(No HTML tags allowed. Separate paragraphs with a blank line.)