Analog Multiplexer

There are a number of possibilities. One that fits the requirements is something like a multi-input analog card for a PLC and a multi-output card somewhere else, linked via a PLC by data cables, with a modicum of signal processing code within the PLC itself. Please describe the context in greater detail for more comprehensive suggestions over solutions.

I have 8 thermocouples measuring the temperature at a single point. 8 have been used for adequate redundancy. These temperature signals are transmitted to far off distances.So its necessary to convert them to 4-20. At the reception site, I want to connect one of these eight thermocouples to a temperature controller. Since all eight signals cannot be connected to a temperature controller, I require a Analog Multiplexer which is digitally or serially addressible. The remote site where the thermocouples are installed are not accessible.

A PLC with 8 remote inputs and 1 remote output still fits the spec as a possible solution. Further, the controller could be within the PLC too, saving hardware.

It is rare to have measurement redundancy at the 1 duty, 7 standby level!

Were the scheme to be carried out, a PLC-based system could also easily analyse the 8 input signals for out-of-range and out-of-tolerance with appropriate coding, and report to some human-machine interface of the need for servicing, whereas a simple multiplexer wouldn't do this. It could also trip the plant off should the level of confidence in the signals drop below acceptable limits, though it would be better to have a hard-wired trip for process-critical temperatures. More often than not, one signal via some form of electronic system and one hard-wired supervisor would be the preferred route.

For 1 duty, 7 standby the reliability level of the selected sensor must be called into question. Also, if the process were that critical, some need for redundancy in the analog multiplexing arrangements ought to be called for.

What on earth does the Functional Design Specification for this plant look like? There's more here than meets the eye......

Thanks for the insight. The eight sensors are at a two different places but essentially measure the same temperature. Further they are brought to the control room through four different routes preventing four anticipated failures.

So, if the cabling exists already, why is a multiplexer needed?

Why not use relays driven by a 3 to 8 line decoder chip?

John.

Is such a unit readily available in the market?

Sensitive relays are easy to get hold of and a TTL 3 to 8 line decoder chip is peanuts and off the shelf!!

The TTL outputs should drive the relays directly, just a few bits around to suppress spikes and regulate volts and its done!!

John.

An 8 way rotary switch would do it....you don't specify how you want it controlled, so I've gone the 'simple is best' route

Hi Frnd

Explore the possibility of using Emerson 848 T tansmitter( FFBUS)

you can connect ur 8 input(t/c ) to one module which can be located in junction box near ur measuring source and from the module you may take digital connn ( using modbus

Regards

Jose