Questions on 7400 +7410 ics

Thread Starter

Earl Wright

Joined Jul 18, 2021
27
I have a circuit board on a 70's pinball machine. Its the logic for scoring from the playfield, among other things. I took a college course 15 years ago and digital was one of the items covered. I remember most of the material, but I was always confused (and still am) about combining several ics to make circuits work, concerning highs and lows. is there anyone here who can give a hand with this? the circuits are fairly simple, 2 ics.
 

Papabravo

Joined Feb 24, 2006
16,806
I have a circuit board on a 70's pinball machine. Its the logic for scoring from the playfield, among other things. I took a college course 15 years ago and digital was one of the items covered. I remember most of the material, but I was always confused (and still am) about combining several ics to make circuits work, concerning highs and lows. is there anyone here who can give a hand with this? the circuits are fairly simple, 2 ics.
Sure. What do you need?
 

Thread Starter

Earl Wright

Joined Jul 18, 2021
27
Here is a schematic. There are lots of these combinations of ic's here. This is a snip it of one. I know that they are 2+ 3 input NAND gates but the purpose and funtion here whats bothering me. There are 2 inputs, one output, a common low for all of these to 'turn them on' I think. Capture.PNG
 

Thread Starter

Earl Wright

Joined Jul 18, 2021
27
There is a cap between outputs. line 13 is labeled logic board reset and I think it goes low to finish the logic on the inputs, since it is looking for a low input on the pins. ? The inputs are definitely low when active (switch to ground on activation of playfield switch) maybe not all the information you need, I can elaborate more if needed. What I do need to know is what happens to the output on low inputs on each ic pin. They trigger separately.
 

Thread Starter

Earl Wright

Joined Jul 18, 2021
27
So if I’m understanding it right, when the L line is initially low, (at the start of a game for example) it will reset the output low on all of these configurations on the board, then when a low comes in on the top ic, it will set the output high, and remain so until a low comes in on the bottom input? such as an s r flip flop?
 

MrChips

Joined Oct 2, 2009
24,198
If you rotate the lower NAND gate you can see the positive feedback action better.
This is a SET-RESET flip-flop. A logic-low level on the /SET or /RESET input is required on one input while the other is held high in order to change the state of the output.

S-R flip-flop.jpg
The extra input L is a means of providing a MASTER RESET signal to multiple flip-flops in the same circuit.
S-R flip-flop2.jpg
 

Thread Starter

Earl Wright

Joined Jul 18, 2021
27
Thank you all for the help and info, some of my older cobwebs are clearing. I will check some of these tonight and get back to all. Earl. I have one other simpler question about an 8 input or gate. ill have to look at the schematic and ask my question.
 

Thread Starter

Earl Wright

Joined Jul 18, 2021
27
After poking around with my digital probe last night, some things are falling into place. Here however is another item i need a little help with. The 7430 should be an 8 input nor gate. ? The inputs, when activated, are ground. Normally, with no switch activation, led#27 (circled) is on. Upon switch closure, the led goes off. The booklet says its supposed to be off, and come on upon switch closure. All inputs work, so should the led be on or off with switch closure?Capture.PNG
 

MrChips

Joined Oct 2, 2009
24,198
7430 - is it a NAND gate or is it an OR gate?
It can be either depending on the context.
SN7430 is listed as an 8-input positive NAND gate.

1626785074601.png


Demorgan's rules state that AND gates can be equivalent to OR gates depending on the context.

Have a look at this diagram:

1626785247467.png

U1 and U1B are two ways of drawing an OR gate.
U2 and U2B are two ways of drawing an AND gate.
U3 and U3B are two ways of drawing an INVERTER (NOT gate).

What do the bubbles (tiny circles) indicate?
Bubbles represent INVERTERs or NOT gates.
They also represent something known as NEGATIVE LOGIC.

What is the difference between POSITIVE LOGIC and NEGATIVE LOGIC.

POSITIVE LOGIC means that it takes a logic-high to activate the function. We also call this ACTIVE HIGH.
NEGATIVE LOGIC means that it takes a logic-low to activate the function. We also call this ACTIVE LOW.

Let us use the AND gate example, U2 and U2B.
U2 is an AND gate for positive logic signals. Active high levels on input-A AND input-B produce an active high output.
U2B is an OR gate for negative logic signals. Active low level on input-A OR input-B produces an active low output.

Let us look at the 2-input NAND gate in U5 and U5B.
For U5, positive logic input signals when ANDed produce a negative logic output.
For U5B, negative logic input signals produce the OR function with positive logic output.

This is what we have with the pin-ball implementation using the 7430 8-input NAND gate.

Switches are commonly wired on the ground side using a pull-up resistor (left side drawing below).
1626786286486.png
Vout is normally high with a normally open (NO) switch. When the switch is pressed Vout signal goes low. This is a negative logic input. Hence our 7430 becomes an OR gate with negative logic inputs and positive logic output. Any switch pressed produces an logic high output.

The LED in the pin-ball machine is wired on the high side (using a low side driver in the drawing on the right, below). The driver has to sink current (active low) in order for the LED to turn on. Hence the LED on the high side is indicating negative logic. Confusing, isn't it?

1626786610334.png

Thus, with the 7430 circuit, the LED is ON when no switch is closed. The LED is OFF when any switch is pressed.

In summary, the 7430 is an OR gate with negative logic inputs (active low) with positive logic output (active high). The LED is wired in such a way as to turn on on negative logic (active low), i.e. the LED is inverted.

btw, the person who drew the schematics for the pin-ball machine in post #13 was sloppy. It shows U18, 7404 as a buffer instead of an inverter. It is missing the bubble needed to represent a 7404 NOT gate.
 

Thread Starter

Earl Wright

Joined Jul 18, 2021
27
Yes it is confusing, so it is operating correctly, despite the incorrect booklet operations. I will check the 7404's tonight to see if they are really an inverter. I forgot to mention in regards to post 15 the pull up or pull down configuration on the front part of this circuit. By looking at the schematic, am I right in assuming it is pull up? 24 is the ground switch. I now get a look at the R-S flip flop that locks on when the switch is made(ic 9), reset on pin 10 and turns on a light, then goes to the 8 input and gate. these seem to be working fine.Capture.PNG
 

MrChips

Joined Oct 2, 2009
24,198
No need to check 7404. We know that they are inverters, one of the most commonly used TTL ICs from the 74xx series besides 7400, 7408, and 7432.
 

Thread Starter

Earl Wright

Joined Jul 18, 2021
27
Yep, your right, they are. This area is working fine, thanks. Now the original issue for consulting the smart people on this forum. Once the logic board reset has been cleared, (the ball goes into play), the 10s and 1,000s digits on the score display start to add up. I have traced the pulses to a 555 ic that is pulsing at what seems to be the same rate as the digits. This ic only starts to run after the reset is cleared, freeing the scoring to accept points. I am still digging into trying to understand what is happening, and I’ve traced a few other lines back, but it gets complicated. It’s looks like it’s definitely coming from this board, as the 10s and 1000s leds that feed the display are pulsing on it. The 555 is involved with a few decade counters, etc, for 500 point and 50 point scores. Sorry for the long winded story, I’m going to dig some more, and if I need any more help, I know to tap the knowledge of this forum. Earl
 

MrChips

Joined Oct 2, 2009
24,198
Noise on the power supply lines.
You have three solutions available. You can try one or you can try all.

1) Find all your LV (low voltage) supplies. Document how they are provided, how they are regulated and filtered. All LV supplies need filter capacitors. Find all the electrolytic capacitors on the supply rails. They may need replacing.

2) Find and document all the 555-timer ICs. Write down the full part number of every 555-time IC you can find. These need filter capacitors across the power rails. Add a new 10μF electrolytic capacitor between power and ground rails at every 555-timer IC.

3) Change the 555-timer IC. Timer chips with part numbers NE555 and LM555 are notably more noisy because they are TTL versions. Replace them with CMOS versions, LMC555, TLC555, ICM7555.
 

MrChips

Joined Oct 2, 2009
24,198
It would seem that you have access to the circuit diagram.
Rather than showing us sections of the diagram are you able to post the complete diagram or a link to the diagram? With this we will be able to better guide you in the trouble finding process.
 
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