9's Complement BCD Thumbwheel Switches

[Tom2000]

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.

 

[Tim Wescott]

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?
-- 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

 

[Tom2000]

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.

Thanks, Tim. That makes sense.

Tom

 

[Mark Zenier]

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?

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 [email protected]
Googleproofaddress(account:mzenier provider:eskimo domain:com)

 

[Tim Wescott]

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 [email protected]
Googleproofaddress(account:mzenier provider:eskimo domain:com)

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

 

[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.

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