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Baltic - ZX-Spectrum-compatible computer, developed in end of 1980-x.
I do not know the place where it was created computer "Baltic", however it was popular in Minsk city due to fact that it was produced and sold by company "Sonet" (I’ll talk about this company later in the article). An early mention of the year of development is found in firmware (1988 year), and disk interface scheme developed in 1988.
A bit of history
The text is based on the memoirs of a former employee of the company "Sonet". Attention! I am not sure of the absolute accuracy of the information and dates (many years have passed and memories may be inaccurate).
In Minsk city computers "Baltic" were produced since 1988 by small company "Sonet". One of the founders was Boreisho (he published some articles about the "Baltic" computer in the journal named "Radiolubitel"). Company store was situated at Amurkaya street. A place of manufacture (assembly area) was situated in school building at Mavra street.
Early models produced using hull unofficially called "heel" (the shape of the computer resembled a shoe heel):
Later computers produced using flatter hull with external power supply:
"Sonet" was produced some models of computer:
"Baltic" with CP/M sold poorly (high price). Was produced from 50 till 100 pcs.
For special orders company was produced models with Beta Disk Interface. Supposedly disk interface could works with TR-DOS and with CP/M OS.
At assembly area worked two women (board solding), two or three adjusters, and one peripheral developer.
Board ordered at Minsk PCB plant. Sometimes the boards come across with defects (wire break).
One day, a batch of boards arrived that worked very poorly. After long attempts to fix them, some of the boards were sold, the rest had to be taken apart.
ROM chips programmed with homemade programmer using "Baltic" :) Thermal training was carried out by setting the ROM chip on the board and left the board on for two days.
ROM chips (2764) with CP/M first had protection with "floated bits", but later protection was removed due there were problems with setting up computers with this ROM chips.
The company "Sonet" made computers until 1995-1996.
In addition to assembled computers, the company sold separately empty boards for self-assembling, computer hulls, floppy interface boards, ROM with firmware and floppy disks with software for "Baltic".
Also boards for "Baltic" could be bought at the local radio market.
In addition to the company, a lot of students BSUIR were involved in computer assembly in the period 1990-1995. Also, the computer was assembled by many amateur radio enthusiasts throughout the country. In the Minsk radio market in those years, you could buy everything you needed to build a computer "Baltic".
A few photos of different computers "Baltic"
Please note that there is no parallel port (82c55) on the board. In its place, a chip (КР1533ТМ8 - analog of 74ALS175) is installed to form the output video signal with gradations of brightness.
Most likely this computer was also assembled by the company "Sonet". On the startup screen is the company logo.
Another interesting "Baltic". Prefabricated board, homemade keyboard (and made much earlier than the computer board).
Mention in the press
Articles describing the computer were published in the journal "Radiolubitel" since first issue (№1 1991).
Journals ″Radiolubitel″ with ″Baltic″ articles
Computer ″Baltic″ advertising in journal
Computer ″Baltic″ advertising in journal
Disk Monitor and ROM-disk
By pressing the buttom "ROM-disk" ROM page with BASIC is disused and instead it used additional page with ROM-disk. From the start menu, you can select one of the programs.
This is one of the options for the start menu (in different versions of ROM there could be different lists of programs on a ROM disk).
An interesting feature of the ROM-disk startup scheme: when you press the "ROM-disk" button for a short time (about half a second), the ROM bank with the ROM-disk should turn on and the computer is reset at the same time. In this case, when resetting, the contents of the ROM disk must be copied to the memory before the ROM bank switches back to the "standard" ROM.
Here is the proposed scheme from the journal "Radiolubitel":
n this scheme, the time interval for turning on the ROM bank with the ROM disk determines the discharge constant of the RC chain. I was not able to achieve a stable and bug-free inclusion of the ROM-disk according to this scheme, so I decided to complicate the scheme a bit, but as a result, the ROM-disk started to turn on perfectly:
DD1.1. removes the bounce of the button contacts. DD2.2 generates a short reset pulse, DD2.1 generates a long pulse (approximately 0.5 sec) of turning on the ROM bank with a ROM disk.
If you press any unused key in the ROM-disk menu (for example, Enter), then Disk Monitor will load into the computer’s memory from the address #0000. This will cause a “normal” reset of the computer to BASIC, but now when you press the "NMI" button, we find ourselves in Disk Monitor:
Disk Monitor is a regular monitor with an appropriate set of functions that allow you to view, edit, save and load data from a tape or diskette (if you have a disk controller). On the Return command, you can return back to the interrupted program and look at the results of work. Thus, Disk Monitor is very similar to the Scorpion ZS256 computer shadow monitor service.
It is highly desirable to correct the formation of the INT signal (a description of the refinement is in journal publications). Without this, some programs might not work on "Baltic", in particular, the TF-Copy copier from the "Baltik" ROM drive does not work.
Work with CP/M
The CP/M system was adapted for usage with "Baltic". System worked at 64 characters per line. To work with CP/M, a disk controller was required. The controller circuit is published in the journal "Radiolubitel".
In various versions of the drive controller, it could work with either one or two disk drives. The controller circuit was very simple, and therefore not very reliable. The output signals were not buffered on open collector elements. However, all this did not prevent the drive controller from working more or less normally :)
The drive controller was originally created as an external device that connected with Baltik via a 25-pin DB-25 connector.
This is how the screen looks when loading CP/M in "Baltik":
For CP/M on Baltika, a re-flashing of the K155RE3 chip with the index "D" is required. The problem lies in the incorrect data on working with the screen area in CP/M. With the "old" version of the firmware, when loading CP/M, the screen had a "mess" of colored dots:
ROM images for "Baltic"
For "Baltic" there were several options for ROM images. I will give here all the firmware that I happened to see:
16K ROM images with Basic-48:
32K ROM images with ROM-disk and CP/M loader:
"Baltic" and Т34ВГ1 chip
According to unconfirmed information, the "Baltic" computer circuitry was used as the basis for the design of the chip Т34ВГ1.
In favor of this version is the fact that in T34VG1 and in "Baltic" the CPU clock frequency is 4MHz.
RAM expanded well to 128K with the help of a “standard” expansion board and soldering onto another RAM memory chips another floor of RAM chips.
Documentation for "Baltic"
I did not see the normal Baltika scheme, except for the one published in the magazine. But thanks to the guys from the site http://sblive.narod.ru a bug-free scheme reconstructed using existing boards appeared. Based on it, I drew the diagram in a more or less readable manner according to standards (you can find link to the diagram at the end of this page).
My "Baltic" with CP/M
For a long time, I believed that the firmware for ROM 27256 with Disk Monitor and the floppy disk with CP/M for Baltika were lost (there was no ROM dump in the magazine, and it had to be bought from the company). None of my acquaintances had this or had been thrown away long ago. But there were good people (rl76 and Backa) who were able to share both the firmware and the disk image. Thus, it became possible to repeat the version of "Baltic" with CP/M: