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9's Complement BCD Thumbwheel Switches

I'm helping a guy with a microcontroller project.

He's going to use some BCD thumbwheels to set a time delay. He purchased some at a local surplus store for a buck each. But they were 9's complement thumbwheels, not standard decimal-coded switches.

I'd never heard of 9's complement switches before, but from the truth table he sent me, I found that decoding them was no problem. (See below)

My question is this: what sort of equipment might have used these switches, and what generation? I read about that system (and complement arithmetic in general) on Wikipedia and surmise that they might have fed a decimal adder, or somehting like that.

Has anyone ever encountered these, and on what sort of equipment? What were they used for?

Thanks!

Tom

====================

Reference:

Decoding 9's complement thumbwheels

Truth table:

1 = closed switch 0 = open switch

Switch Setting 8 4 2 1 Complemented Decimal

0 0 1 1 0 9 1 0 1 1 1 8 2 1 0 0 0 7 3 1 0 0 1 6 4 1 0 1 0 5 5 1 0 1 1 4 6 1 1 0 0 3 7 1 1 0 1 2 8 1 1 1 0 1 9 1 1 1 1 0

Connect each switch common to ground, then connect the 8, 4, 2, and 1 switch terminals to the microcontroller using pullup resistors. (That active-low arrangement automatically inverts, or complements, each switch condition, completing the first step in the decoding process.)

Read each thumbwheel as a BCD nibble. Subtract that reading from 9 to arrive at the decimal switch setting.

Reply to
Tom2000
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-- snip --

Something like this would be of use if you were preloading an up-counter like, IIRC, a 7490 or 74190. You load the counter, then count up until you get a 'carry' output (at 99...99), and voila!, you've counted out the number on the switches.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Thanks, Tim. That makes sense.

Tom

Reply to
Tom2000

Digging out a Fairchild TTL Applications handbook from 1973, they show they could be used with the 9310 decimal counters for multistage programmable counters. (When Texas Instruments did their version they numbered it the 74160). The 74160-63 family are synchronous up counters with preset inputs. There is logic in the chip to feed the clock control inputs of the next counter in a chain. All chips in the chain get the clock pulse in parallel, with a couple of clock control inputs that enable the count, or load from the inputs.

The trick for using a 9's complement input value is that there's an extra Nand gate needed that decodes when the counter chain is at 9...98. When that happens, instead of the counter going to 9...99, the counter chain gets set to the value coming in from the switches. So the count runs from the complement value up to the 9...98. Looks like it might get a bit wierd if the switch setting is 0...00.

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

Reply to
Mark Zenier

For most things (certainly if it were used in a frequency synthesizer) the first few "digits" would be fixed, with only trailing digits being adjustable. That would take care of the '000' case.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Thanks very much, Mark, for the time you spent researching this. And I'm amazed that you still have 1973 data books on the shelf! (Amazed in a jealous sort of way. I wish I'd kept a lot of stuff from that era.)

That's sort of the circuitry I was expecting after I'd read that Wiki article.

Best,

Tom

Reply to
Tom2000

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