Industry isolated communication

You haven't really told us enough but if you can get away with RS485 speeds why not use a fibre optic link.  Plastic fibre and associated parts are quite cheap and easy to use  - you would need one twin fibre and there would be no trouble reaching decent baud rates. You can use the UART on pretty much any micro connected direct to the fibre driver chips. You could use little FPGAs for less latency (if it matters).

MK

What are the two devices, what voltge levels are we dealing with and how far apart are they, as Michael has already stated, more information would be very handy to enable us to help you better

First of all, thank you the replys (also Michael) !

Sorry the little information, but I did not know much about the situation. The voltage level is 24VDC, on both sides (1 robot and 1 CNC machine). I think the speed non time-critical (so no need a many MHz...). The distance I think some meters, but I inquire this.

Edit: the distance is 25 meter.

It would probably be worth building a parallel to serial adapter at each end to send the 8 bits down a differential pair (RS485), this will minimize noise and allow plenty of didtance to be covered (More than enough for 25Meters)

you will need to drop the levels to 5V probably then into a shift register or a simple micro-controller with a serial interface (Arduino UNO would be a simple choice, or if your buyilding your own boards, base it on an ATMEGA328 so you can esily prototype it with an UNO at each end

to go from 24V to the 5V, a simple opto isolator chip would work (4 channel device * 2 at each end). A RS485 tranceiver would be used between the two UNO/ATMEGA328s for the main comms

I am more skilled in the Microchip PIC microcontrollers (but I think this irrelevant).

So the basic theory:

- For the 8 channel use 1-1 MCU, and between two MCU use a RS485 interface.

- In "Tx PIC" the program polling (or handle an interrupt) the 8 inputs, and if detected any difference compared to the previous state, then send the actual state on serial interface (trough RS485).

- In "Rx PIC" the program received the serial data, and put that to output (8 pin).

- ... and vica-versa.

Opinion?

Thank you!

Exactly, and yes you are correct, any micro will suffice and if your familuar with PIC, then go for it, it will work as good as any other for this application

In transmission you can try sending the byte, then the XOR of it as a check, that way there is no need for a return except for acknowledgement and as the serial speed can be fast, this will not impeed performance, this all depends on how improtant it is for 100% accuracy, other mechanisms can be imployed like a message ID etc added to transmission and the ack can be an XOR or other checksum of the entire package

Regards

Peter

You mention isolation for your serial comms and that is a good idea in an industrial environment. Recently used ADM2582E with good results for an isolated RS485 interface.

If you really want isolation then you are much better off using plastic fibre rather than RS485 - 10's of kV of isolation, very good EMC, no earth loops/common mode issues.

MK

and from a non optical approach, TI has some isolated tranceivers for RS485 for a few $$$ each

http://www.ti.com/lit/sg/slyt484a/slyt484a.pdf

these provide full galvanic isolation upto about 4KV

Either way you still need the Parallel to serial conversion of your data or many isolated traceivers, it is how you get from location A to location B that is the descussion here

Fiber will add a couple of hundred $$ to the cost or more depending on where you shop and may need ethernet as the connection at each end which is also a viable option btw and ethernet also provides isolation due to its termination transformers at each end of the cable

Ethernet will provice about 1000V of isolation on its own

from a cost, Ethernet and RS485 should be the cheapest, optical more expensive but providing superior isolation if needed

Plastic fibre will add far less than $200 to the cost and no way does it need ethernet.

You can connect low cost optical transceivers directly to the UART ports of the micro.

TX HFBR-1522Z (Farnell 1173129 £8.80) - will need a driver chip but it's not expensive

RX HFBR-2522Z (Farnell 1173132 £8.88)

The data sheet from Avago is helpful on these parts.

20metre coil of cable (Rapid Electronics 58-0710 £20.81)

Ethernet (using the usual cables and transformers) is an option but needs much more software and more expensive and complicated micros at each end. There are some TI launchpad thingies with Ethernet built in which might make a simple way to go.

(BTW Ethernet transformers are supposed to be tested for 1000V breakdown - they are not usually rated for safe use at 1000V and most pcbs are not tracked to provide serious HV isolation. RJ45 connectors may or may not comply with the regulations for creepage distances so any solution with magnetics built into the connectors should be considered as an earth loop avoidance solution but no good for safety isolation (unless you have evidence that they are OK))

100Mbit Ethernet using only basic Ethernet protocol (no IP stack or RTOS) can give very good performance at low cost - any kind of OS and using off the shelf IP software will add a lot of latency and slow things down significantly.

MK