Help 14 Led display

[eskymos]

Hello
I want to interface 6 led's that have 14 segments to a 8279.
How can i connect these to the output ports?
There are only 8 output ports for the display.
I have read the datasheet.
8279 datasheet pdf datenblatt - Intel Corporation - PROGRAMMABLE KEYBOARD / DISPLAY INTERFACE ::: ALLDATASHEET :::

Please help

 

[stefanodel]

Hello
I want to interface 6 led's that have 14 segments to a 8279.
How can i connect these to the output ports?
There are only 8 output ports for the display.
I have read the datasheet.
8279 datasheet pdf datenblatt - Intel Corporation - PROGRAMMABLE KEYBOARD / DISPLAY INTERFACE ::: ALLDATASHEET :::

Please help

I am very surprised to hear that someone wants to use this type of IC. I used this component in 1983 just to drive 14 segments display. Today this IC is obsolete and I don't know if you have in your hands the 8279 or you think to buy it.
Anyway to drive the display I connected the ports B0,B1,B2,B3,A0,A1 (from pin 26 to pin 31) to the address bus of two eprom 2716 (A0,A1,A2,A3,A4,A5) in order to decode the fourteen segments of display.
I know that someone can laugh hearing that components I have used, but it was more than twenty years ago.
I do not know if it is worth the trouble to use the 8279, by now forgotten from all.

Stefano

 

[duffy]

It may be possible, depends on how the 14 segment displays are wired. What you would do is drive each with 2 scan lines - one for 8 segments, then again for the remaining 6. You will have to set the 8279 for "encoded" mode and add a 4 to 16 decoder to get the 16 scan lines, 12 of which you would use for the 6 displays. You will also need drivers, that things not designed for direct-drive capability.

 

[duffy]

Can't I just connect the output ports OB0-OB3 to a 4 to 16 decoder
and use the first 8 outputs to select the first 8 segments in a led, and the next ones to select the other 6 segments?

Also, can you tell me where exactly do i have to connect the scan line to select the first 8 segments?

Why do I have to use 12 scan lines to select 6 displays?
I thought I would have to connect only the cathode pin from a digit
to the output of a 3to 8 decoder (the inputs are SL0,SL1,SL2) in order to select it.

You don't want to hook OB0-3 or OA0-3 to a 4-to-16 decoder because you need all those bits to work in parallel to light up groups of segments simultaneously. If you decode them it will only light one at a time.

You would use 12 scan lines off a decoder (connected to SL0-3) because you need to scan each display *twice*. They are 14 segment displays, too many segments to display at once with that 8-bit (OA + OB) output.

What's more, they will need to be 14 segment displays something like this one -
https://www.electro-tech-online.com/custompdfs/2008/12/MDA6141C6341C6441C6941C.pdf
- with a divided common anode (or cathode) so you can drive half of each display seperately.

I really think you would be happier with something like a MAX6954 instead of that 8279. It was designed to drive 14 segment displays, it has built-in drivers, it's still available (!), has a brightness control register, has the keyboard scanner, and works off a SPI port so it doesn't eat up a lot of I/O.

 

[eskymos]

Can't I just connect the output ports OB0-OB3 to a 4 to 16 decoder
and use the first 8 outputs to select the first 8 segments in a led, and the next ones to select the other 6 segments?

Also, can you tell me where exactly do i have to connect the scan line to select the first 8 segments?

Why do I have to use 12 scan lines to select 6 displays?
I thought I would have to connect only the cathode pin from a digit
to the output of a 3to 8 decoder (the inputs are SL0,SL1,SL2) in order to select it.

Thank you.
I'm waiting for your response.

 

[duffy]

If you hook those outputs to a decoder, it will only light one segment at a time. To create characters on a 14 segment display, you need to be able to light more than one segment at a time. The decoder would go on SL0-SL3. The reason you are using 12 scan lines for 6 displays is that you are doing each display twice, half a character at a time. This means you are going to need a display with split common anodes (or cathodes) like an MDA6141C.

I really think you would be much happier using something like a MAX6955 instead of fighting with this old 8279. The MAX6955 has built-in drivers, a character ROM, display dimming, keyboard scan capability like the 8279, and only needs a 2-pin SPI port instead of that cumbersome parallel bus interface.