Input/Output Multiplexer with Digital and Analog

[BigBill53]

Hi,
I have a system where a number of digital inputs (10 of them) are multiplexed and sent across 2 wires back to the control room. The customer now wants a few 4-20mA signals sent back as well. Running more wires is not an option here - it's going to be too expensive.

Does anyone know of a multiplexer (or some kind of smart device) that handles analog signals as well as digital ones?

I had thought of a possible alternative - an input card that sends the signals back over RS485. In that case I would probably use Allen-Bradley flex I/O, but I don't want to be stuffing around with things like DeviceNet to RS485 converters.

 

[EddieWillers]

You need a common wire or a well-bonded earth for RS-485 to work; just two wires won't do the job. Are these a twisted and shielded pair, or just a couple of stranded wires ?

You'll find wireless I/O transmitters; FreeWave, Banner, Data-Linc, etc, as well as serial I/O multiplexers that will run on RS-485; Automation Direct Terminator and FLEX I/O all have RS485 Modbus adapters.

But not many companies still make modem-based I/O multiplexers.

The big question, then, is what sort of wires these are ?

 

[dV8r]

there was an invention a while back to take care of this kind of dilemma.
I think they called it wireless or wifi, or something like that. Try this:

http://www.bannerengineering.com/en-US/wireless/surecross_web_home

 

[BigBill53]

I'm pretty sure it is a twisted and shielded cable, but without personally viewing it myself I am a bit skeptical. It wouldn't surprise me if they are just a pair of telephone wires. I'll check it out.

Wireless isn't an option here because the two points are separated by a large cliff and dense jungle. You would need an aerial 40m high to get a reliable signal.

And of course, they only rain one pair of wires when they first ran this insanely hard run of cable. People never seem to think about future expansion.

 

[EddieWillers]

Jungle ! My application challenges are always made of hot sand and rock so I'm slightly envious.

Since the two sites are highly unlikely to have a common ground potential, RS-485 is ruled out.

This application says "modem" to me. Good old FSK modems running at 1200 baud can run on a barbed wire fence and still give you modest update rates for a few words of Modbus data.

Because the first shelf of my toolkit is usually Allen-Bradley PLCs, I would grab a MicroLogix 1200 or even one of the new Micro 830 controllers and just use it as a DF1 (MicroLogix) or Modbus (Micro 830) slave.

The hardened Bell 202 style modems I use are the good old Data-Linc LLM-1100's. They take a licking and keep on ticking.

If your system on the other side needs actual physical outputs, reverse the method with another MicroLogix or Micro 830.

Substitute Logo! or S7-1200 or Twido or DL-05 or whatever small controller you prefer.

 

[BigBill53]

Thanks, it didn't even occur to me to use a modem. I've used them plenty of times for transmitting RS232 data over radio - I don't know why it didn't occur to me.

Still, I'd like to have a solution plug and play out of the box. It is a fairly remote site and I need to have something that a monkey could take out of a box, plug it in and it goes. Having PLCs on either end that need to be programmed is too dangerous here, because it is for an entire town's water supply. If there is more than 6 hours downtime the town will lose water.

I found out today that the wires aren't shielded - just a common pair of telephone wires.

 

[EddieWillers]

Years ago Data-Linc made an I/O repeater based on that LLM-1000 modem, but with the increasing popularity and speed of wireless the market for them dried up and they stopped production.

If it were my system I'd probably choose a micro PLC that can be programmed with a pluggable memory module, rather than requiring a download with a software utility. Buy extras, program them, and hide them in the cabinet.

I think you're looking at a tradeoff between using a PLC and modem from a major vendor that can still be bought in ten years, or using a smaller-vendor or legacy system that might be simpler to install but doesn't have long-term availability.

Control Microsystems might have an I/O repeater product that uses their Bell 202 modems and will work in this application.