Multiplexing two-state inputs

Z

Thread Starter

Zvi Marcus

We are using 8x8 DDC controllers. Both inputs and outputs can be programmed to be either digital or analog. The inputs are 0-5 VDC. Outputs are 0-10 VDC.

Most of our applications require more inputs than outputs. As a result we have to add controllers and wind up with unused outputs. The bulk are low speed digital inputs, on/off state of motors, switches and other devices.

We're looking for a reliable way to "stuff" a number of on/off states into one controller input. Time multiplexing could be a solution. The controllers run at least at 20 scan/sec and we could stand a one or two second delay in receiving a change of state. We could even spare an output (0-10 VDC analog or digital) for synchronization if necessary.

We prefer something tested and "off-the-shelf" but price is an important factor.

Thanks for your help.
Zvi Marcus
 
Do you have any impossible input combinations? ie two or more devices that cannot be on at the same time. If so you can connect another device so that when both inputs are on then it must be one of them. That is a simple example, you can get more complex. You weren't very specific, but that is one possible way to solve your problem.
 
S

Steve Myres, PE

I'm not aware of off the shelf solution for this application, but if you are unable to find one, and would like my help with a custom solution, feel free to contact me off-list.

[email protected]
 
S
Take a look at www.phoenixcon.com they offer digital to analog converters that are powered by 24vdc and support 8 or 12 bit input to 0-10v output. The part numbers are 2808190 (MCR-DAC 8/U-10/BUS) and 2786490 (MCR-DAC 12/U-10B/MSTR)

 
We don't manufacture, but I have designed various taylor-made systems to cater for the problem.

Reply if you're interested.
Chris
 
C

Curt Wuollet

why not use a straight binary encoder, 8 lines into 3. I think it's something like a 73ls147 or 148. one is bcd and the other is binary. you would want tbe binary as BCD "wastes" bits. This way you don't have to do any micky mouse muxing. This should be all of $3.00 at the most. You could get 34 inputs on your 8x8 and speed would not be a problem.

Regards

cww
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C

Curt Wuollet

By George you're right, I just looked at the truth table and it's not a full decoder. The principle is good though, there must be a ttl decoder available. I'll see if I can hunt one down to attone for my sin. That's what happens when you rely too much on old memories. I apologize to all.

Regards

cww
--
Free Tools!
Machine Automation Tools (LinuxPLC) Free, Truly Open & Publicly Owned Industrial Automation Software For Linux. mat.sourceforge.net.
Day Job: Heartland Engineering, Automation & ATE for Automotive Rebuilders.
Consultancy: Wide Open Technologies: Moving Business & Automation to Linux.
 
C

Curt Wuollet

Try a pair of 74LS157 and use 1 of your outputs as /select. This would give you 16 inputs. I think that's what I might have been thinking of
in the first place but 147 came up from core and I went with it. There are other muxes if you can spare more outputs. I am appropriately
embarrassed. Another good reason for the PCPLC project, I have been far from my digital roots for too many years. But I've found my wire wrap
tool and an ISA proto board is on order.

Regards

cww
 
P

Peter Whalley

Zvi,

You can build a simple digital to analogue converter using a few precision resistors and a stable DC voltage source. Have a look at some introductory text on digital to analogue conversion techniques for possible circuits.
Each monitored point becomes one of the digital inputs to the converter. You want a circuit where all the input switches are grounded and the
resistive ladder is based on R and 2R value resistors only.

Read the analogue voltage produced using an input in analogue mode and then read the most significant bits individually of the input word to get back to your original digital inputs.

I've seen this done for 2 inputs but you should be able to go to 4 or more with a bit of care.

Regards

Peter Whalley
Magenta Communications Pty Ltd
Melbourne, VIC, Australia
e-mail: peter*no-spam*@magentacomm.com.au
delete *no-spam* before sending
 
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