How did people multiplex these old three digit displays before MCUs? - Page 1
Forum section: electronics / beginners

author: dentaku
author_rank: Frequent Contributor
timestamp: August 28, 2014, 03:05:36 am
content: Back in the days before microcontrollers how were things like this 3 digit multiplexed common cathode display used? I have one and I love the look of it.http://www.ebay.com/itm/ARCHER-3-DIGIT-LED-MULTIPLEXED-DISPLAY-7-SEGMENT-COMMON-CATHODE-276-055-NOS/131261896398?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D24189%26meid%3Df8a6ec7aa6734a6fa80ffca3d31c7cc7%26pid%3D100005%26prg%3D10283%26rk%3D1%26rkt%3D6%26sd%3D390898460603&rt=ncIt has 8 anodes (extra one for the dot) and 3 cathodes (one for each digit). I see how you could turn on and off each digit in sequence really fast using the separate cathodes but what kind of convoluted multiplexing setup would you need to make a simple two or three digit counter, for example, back in the old-er days with one of these displays?I have some old 5 stage Johnson Counter - 7 segment decoders and I just hooked up two of them to 2 digits of one of these 4 digit displays and it works fine.http://www.ebay.ca/itm/2-Archer-276-1202-Clock-Display-Bd-4-5-Digits-w-colon-DI-/320557166141It was easy because there's aseparate anode for each segment of each digitunlike the multiplexed displays.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: August 28, 2014, 03:35:25 am
content: Logic circuits:- Clock generator- Counters- Decoders- GatesDavid.

author: rsjsouza
author_rank: Super Contributor
timestamp: August 28, 2014, 03:37:50 am
content: For the common cathode case, you would simply connect a 4511 outputs to the segments and select each cathode via external transistors. Something similar to what is shown in page 5 of theCD4511B datasheet. A more legible picture is shown in the second diagram ofthis page.For the common anode you would use a 7447 as shown inthis page.

author: sarahMCML
author_rank: Regular Contributor
timestamp: August 28, 2014, 04:29:49 am
content: If you want all three BCD counters, along with the scanning and multiplexing circuitry, etc, on a single chip, you can't beat the old CD/MC14553 CMOS device. Cascadable to more digits as well. Pick your own output drivers, or even use directly with modern very low current LED arrays. Always assuming it's still available, of course!Regards,Sarah.

author: David Hess
author_rank: Super Contributor
timestamp: August 28, 2014, 04:49:05 am
content: Usually the BCD decoder/driver is multiplexed as well so only one 7447 or similar is needed. A counter drives a 1 of x decoder which drives each column iteratively with the correct BCD digit applied to the decoder/driver.Many BCD counters included several digits and implemented the 1 of x column driver and multiplexed BCD output in one chip so only a 7447 or similar and a common segment driver was needed.Take a look at the schematics for any of the old Tektronix TM500 DC series counters or multimeters for an example.

author: dentaku
author_rank: Frequent Contributor
timestamp: August 28, 2014, 04:37:45 pm
content: Quote from: sarahMCML on August 28, 2014, 04:29:49 amThat 4553 looks quite useful.

author: dentaku
author_rank: Frequent Contributor
timestamp: August 28, 2014, 04:43:19 pm
content: Quote from: David Hess on August 28, 2014, 04:49:05 amI see, it's much easier to multiplex the BCD because it's only 4 pins then all you need is a regular BCD to seven segment decoder/driver and somehow to switch on and off the 3 digits in sequence (if you don't have something like a 4553B).

author: jlmoon
author_rank: Unknown
timestamp: August 28, 2014, 05:01:23 pm
content: I have seen 74154's used as column drivers or enablers as well