Two Separate Grounds?

While trying to figure out the display system of an old Solid State pinball machine without a working CPU board, I encountered a mysterious problem. To me, it looks as the system has two different “Grounds”. One for the TTL circuits, parts of the displays and more, and another for the CMOS, the lamps, other parts of the same displays(!), and the coils. Could it possibly have something to do with CMOS working with different signal levels?

In this excerpt, we see that the outputs from the two TTL circuits A5 and B5 (both 7448) are directly connected to the base pins of the TDA3081 transistor arrays A6 and B6 - but they seem to have different kinds of grounds. Not totally isolated from each other I guess, or it wouldn’t work at all as far as I understand. But yet they apparently do not share the common Ground?

So far, I have connected the Arduino, the Multiplexer and the drivers for the playfield to the Ground with the “E-shaped” symbol, but now that I wish to involve the display units, I don’t know if I would dare to just connect these two grounds together. But on the other hand, with the Mainboard already destroyed by a previous owner, there is no CMOS logic left, so maybe it is risk free to connect them. Any advise here would be much appreciated.

The entire system manual can be found at: http://www.flippers.be/Recel_System_III.pdf (3.6 MB). The screen dump above is from page 5-18.

TIA,
SimLego

TTL logic is quite sensitive to ground noise, CMOS logic less so. Perhaps the designers were trying to keep all the high current stuff (lamps, coils etc.) from inducing noise in the ground used by the TTL logic.

Interesting complicated machine.

I would guess that the grounds are all tied together on some terminal block somewhere inside the machine. You can't have them being separate, without some sort of shorting together somewheres. No evidence of optoisolation. There may be a place where they are tied together through inductors for noise isolation - I think that might be a good idea here, no?

What it interesting is the earth ground [upside down trident symbol] is shown on page 4-4 tied to the earth ground wire on the power mains. If that is the same as the earth ground symbol being used on almost every other schematic, that seems a little strange to me. Maybe europe has different grounding standards.

I would simply hook an ohmmeter, and see if the board grounds are all connected, and also connected to the earth wire on the power mains.

To me, the diagram is confusing. I would have used a standard ground symbol on the individual boards, ie upside down triangle with 3 bars or plain upside triangle, rather than the earth ground symbol. Then on some page, showed a diagram as to how they were all interconnected. Otherwise, it's a confusion.

Finally, I found it!

They are connected at the Power Supply.


PC05 (Electronic Ground, Black-Blue wire) to PC06 (Electric Ground, Black wire)…


split up ever since then:
PC05 - MF11 - ME01 - DE10
PC06 - MF06 - ME04 - DE09 - DD01…

I sort if suspected this long ago, but ohmed them and found 30 ohms between them. Now I know why. Poor contact in the abused Molex connector!

I guess that makes sense after all.

Thanks for the input!

/SimLego

oric_dan: What it interesting is the earth ground [upside down trident symbol] is shown on page 4-4 tied to the earth ground wire on the power mains. If that is the same as the earth ground symbol being used on almost every other schematic, that seems a little strange to me. Maybe europe has different grounding standards.

That symbol, in Europe, means 'chasis'. The chasis may or may not be connected to earth/ground. In the UK, the chasis of CRT TV's were never earthed/grounded, so that symbol was used extensively.