Some time ago on one of the popular auction sites I noticed a rather interesting offer. It was a module of unknown origin with a small VFD lamp, which I suspect was the seller’s main motivation to get rid of it. After all, fluorescent designs sell quite well, though not as well as Nixie lamps. As an enthusiast of all sorts of freebies from the days of justly past socialism, I could not let go of the purchase. After a few days, a package with three modules arrived, which I will now describe in a bit more detail.
Looking at it from the outside, one can tell quite quickly that it is a design from the former Soviet Union. This is indicated primarily by the ubiquitous zirilica and the logo on the chips, informing us that the chips come from the former Minsk “Сигнал” plant.
The design itself is quite small, and you can certainly tell that it was part of some larger device or machine. Attached to the laminate are metal brackets that allowed the module to be attached to the front panel or case, so that the display composed of VFD tubes could be seen by the user. On the board we find the aforementioned two ICs, designated 827ИД1-1 and 827ИД1-2, manufactured in December 1985. While their case may look quite ordinary, considering other Soviet designs, it is worth noting that one of the chip’s leads is placed in its central part. This can be seen well on the right side of the photograph, where, in addition to the row-placed leads, a single central pin can also be noticed. Another date marking is also visible here. “XII 86”, may suggest that the module or the laminate itself dates back to December 1986. Throughout the design, we can also distinguish thirteen external leads, to which, as can be seen earlier, wires were soldered.
The single VFD lamp housed in a special metal holder is actually a rather popular seven-segment ИВ-8, from the Saratov plant “Рефлектор”. Its markings indicate that it was similarly laminated and produced in December 1986.
The purpose of these designs remains a mystery, but there are a few details that may suggest military equipment. First is the protective varnish with which the board, as well as the circuits, are covered, it was also covered with it probably after the whole thing was assembled. In addition, you can see glue on the threads of the screws to prevent loosening, which is very rare in commercial equipment. Also notice the red paint on the external leads, probably applied at quality control. The marking of the chips does not indicate a military application, the letter initial in front of the digital marking is missing here, but I have already seen this type of situation, where commercial chips, were used in military equipment. In addition, searching the Internet, I came across a forum where it was reported that these types of chips were used precisely in military equipment, but I have not been able to confirm this information in any way.
Launching the module
Interestingly, the control of this module is surprisingly simple. As I mentioned, there are thirteen pins at our disposal – two of them (connected by a path on the board) are used to connect the ground. Right next to it is a pin for the lamp glow voltage (about 1.2V) and the anode/grid voltage (about 25V). The remaining nine pins are used to select the displayed character. And so it is nine pins, although the supported characters are ten – from 0 to 9. The displayed character is selected by connecting the corresponding pin to ground, and when none is connected to it, the display shows the number 8.
So, as you can see, the module is a simple controller/decoder of code 1 of 10 (almost) into signals for a seven-segment lamp. This is interesting and a bit surprising, because usually this type of display is controlled using BCD code.
Diode logic
Let’s stop for a moment at the chips themselves, because these are not such ordinary chips. Analyzing the 827ИД1-1 and 827ИД1-2 designations more closely, we see the numeral two in the second place, this means that we are dealing with hybrid designs (by the way, if you would like to see an article describing how Soviet chips are designated let me know via my Instagram). The designation agrees, it is a hybrid chip made in diode logic. Therefore, hidden under the green protective glut, the cores are in fact semiconductor diodes and, importantly, a single structure can contain even several of them. They are attached to a ceramic substrate with paths connecting them to the circuit leads.
You can see the inside of the diode structures in the diagram above. As you can see, it is nothing complicated. The ground attached to the cathode of the diodes during the control of the module makes the diodes properly polarized and the given line is shorted. On the other hand, when the diode (hangs in the air) and no voltage is attached to the control lead, a supply voltage is maintained on the VFD lamp control line. As you can see, the circuits are quite simple, and you may have run into the same question I did, why are these two separate circuits and not one?
It turns out that originally this chip was indeed a single chip, designated simply as 827ИД1 without the additional digit. A photo found on the Internet suggests that this was still the case in 1977, but why, over time, did Soviet engineers decide to split the design into two? That is likely to remain a mystery forever.
