Logic behind multiple driving seven segment displays

[ant.merlino]

Hi,

I am trying to drive multiple 7 segment displays. The application is basically a price sign which will need to be able to count up and down. I have looked at a companies take on the design and I'm a little confused. They are using octal (which I understand why in a way) with shift registers to drive the displays and communicate between the two. If the application is just to be able to count up/down and display numbers, why couldn't you just use a bcd to seven segment driver and a BCD up/down counter with a clock pulse coming into the rightmost digit. Then the carry bit of the up/down counter would be the clock pulse for the second digit and so on? Am I missing something here or would this work.

Note: This application will eventually be communicating with a remote to set the number. I do not know if that has any effect on the companies choice to use octal.

Thank you for any help/info

 

[crutschow]

Not sure why they would use octal if you need to display 0 through 9.

I would think you would use a BCD up/down counter, not binary or octal.

 

[ant.merlino]

Sorry that was a typo on my part, I meant BCD not binary. But thats what I'm trying to figure out. Am I missing something? or did they just make it more difficult then necessary?

 

[KeepItSimpleStupid]

Let's say this is a gas station price sign. Multiple displays, remote. For fun, lets say that the operator cannot see the outdoor display. For now, let's assume 7-segment, but it's probably a 5 x 7 or larger DOT matrix made of conventional LEDS and it needs a brightness control.

Now let's do it the old fashioned way:
Remote 5 X 7

Use a BCD to decimal thumbwheel switch, Load the values into a shift register. Create a start bit for the group and start sending data to the remote sensor. Let the outdoor display deal with the brightness.

New way might be make the sign into a webserver. Login to the webserver using ssh and change the price. The price would change when the corporate office says it should. The price is reflected immediately into the POS terminal and the gas pump reflects the change too. One place has to change the sign and the individual pumps and the POS terminal.

Basically it makes sense for each system to take the value from a database.

Now what happens when power goes out? Well, you used some ferromagnetic memory which doesn't need a battery.

Now what happens when the database goes down for maintainance?

Think:
HMI
COST
INTERCONNECTIVITY

Have you ever seen the LCD displays that are attached to items to be sold at the store. The price as well as various things such as "sale" appear on the battery operated display. I have no idea how they work. Kohls has them.

Who says "It has to count"? Octal is a representation. 8 bits fits into a standard UART and that may be why.

An rotary encoder would be today's way of entering data. Have a local display/lcd module. Maybe a "LONG PRESS" would advance to a display and the display would blink. A short press would blink all digits b.bbb, but the one your changing. Push. 3.bbb, Use encoder to adjust the 3. etc. It would start with the last price as input.

When all of the prices were adjusted, the display would blink for 5 sec. and then shutdown. A tap would turn on the local display for 20 sec. Brightness controls would be used as well.
Vacuum florescent display would be used for readability in light or dark situations.

Thinking out loud.

 

[ant.merlino]

ironically, this is for a gas station price sign and they do use a dot matrix of leds for each segment. A friend installs signs for gas stations. Usually does the plastic signs but now that we're moving into a more digital world he has been forced to take on some of these jobs. He got some units for cheap but they are junk. They flicker, are hard to set, and constantly have problems. Unfortunately the other displays are much more expensive and don't leave room for profit. I helped him with the cheap signs and struggled to get them working. I told him I think we could build are own units if we took the time and did it right. They'd be quality and still cheap. The best displays on the market are simple. They have a remote with 4 sets of up down arrows (8 buttons total, an up down button for each product regular plus super diesel) Nothing fancy, and honestly it doesn't need to be. The digits count together meaning the smallest placeholder counts until it carries to the next and so on. No setting each digit of the price independently. As far as memory, I don't think you need it. If the power goes out at a gas station (which is rare seeing how they usually have back up generators) It would not be difficult for the prices to be reset. Point being, what is the advantage in using the octal system with shift registers (can you explain in detail how that system functions). What are the downsides to building the simple circuit I described?

Thanks

 

[KeepItSimpleStupid]

The "shift registers" is nothing more than "parallel to serial" conversion. If at one end, I had the BCD version of each digit loaded into a shift register. No big deal, right. So I have 5 nibbles of 4 bits. Call it hex, call it octal. I don't care. It's 16 bits. Fixed decimal place.

Octal fits in standard UARTS. Maybe they packed it into 2 UARTS and they clocked the data out.

Maybe they had their own 16 bits of info, created a start bit and a stop bit, and sent the data out asynchonously.

The whole point is to use a few wires as possible to communicate to the display. Using existing wires or wireless is even better.

What kinds of signals do you need at the display end?

Where is the engineering problem with the current system?

How much does the current system cost?

What are they willing to pay for a retrofit controller?

What are the typical wires that are used for communication? Both type and function?

The simple circuit could be part of it too, but with the prices of microcontroller so low and glue logic so hard to find why not go microcontroller based.

Although I might not like it, because of expense, it's surely possible to design a sign system using say a Smart relay type of PLC and a MODBUS slave device. https://www.electro-tech-online.com/custompdfs/2011/06/14105EN.pdf and a little bit of glue.

It won't be anywhere near cheap, but it could be easy to do.

This https://cubloc.com/product/08_01.php isn't too bad. It had MODBUS and programmable in C, so it's a possibility.

Maybe add a 16 bit MODBUS output module like this one: https://www.omniflex.com/otspecialfunctions.asp You have 16 digits x 4 displays. You may not be able to multiplex that fast, so your glue might be a module with a latch for each digit and a brightness control and whatever power board issue you have.

I'm just thinking outloud here with not much to go on. Also check out www.cpustick.com for some interesting wireless options.

 

[KeepItSimpleStupid]

Here is just one such system: https://www.scoreboards.net/gas_price_signs.htm

Basic: handheld controller or Internet. Powerline communications. Forgot about the 2 directions.

 

[ant.merlino]

ok that makes sense. but at the same time wouldn't this be a better solution. The remote sends one signal from the button instead of a series of 0's and 1's. The signal is received and depending on which button was pressed, the appropriate clock pulse was switched on to the up/down counter.

As far as a microcontroller, I don't have experience with programming them. I can program. and I understand the logic. but I never actually programmed one.

And thats the thing. I don't have much to go on either. I'm just trying to find the simplest solution that can be prototyped into a cheap and easy to use design that will be well made enough to survive the installation and last

 

[KeepItSimpleStupid]

Not having any info makes things really tough.

Your not dealing with a BCD to 7-segment display.

Remotes, are getting to the point that they require microprocessor interfacing. The is at least one company that makes RF modules that don't have to be FCC certified. https://www.linxtechnologies.com/

How long will it take to count to 400 or $4.00/gal without some sort of fast incrementing logic?

What about stations with multiple signs for the same grade? Probably rare, but I bet it happens.

Counters and RF does seem pretty simple until your realizing that your not dealing with a BCD interface. It's probably a matrixed display that's scanned. Not running extra wires is a big plus.

If your in the business of retrofitting signs, then it's in your best interest to select a trouble-free product. If you wish to retrofit an existing sign because of poor reliability first, you have to convey that to the manufacturer so that they can address it. If the manufacturer went out of business, then you may have an opportunity. Using commercial parts might not seem that bad of an option then.