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SMEMA standards


SMEMA standards | 18 September, 2002

Someone that makes conveyors just called me to argue about a smema problem we had with his conveyor and our machine. He caught me off guard and I wasn't the person that handled the problem so I stumbled through this initial conversation. Now that I was able to get some info I'm ready to call him back.

Here's the senerio: Our machine uses opto couplers and his conveyor uses some kind of mechanical contacts, these contacts are not dependent on polarity while the opto is.

He claims that the smema standard 1.2 allows him to wire pins 1 & 2 any way he pleases. I feel that the machine we are interfacing it to is wired correctly (pin 1 signal, pin 2 return). After looking at the spec I can't find anything in writing that says what pin 1 & 2 are, but there is a diagram that clearly shows that the "machine not ready" signal is pin 1 and 2 is the return.

Before I start the fight I just want to verify that I am correct. Any smema experts out there that can back me up?

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SMEMA standards | 18 September, 2002

You can find the spec on the IPC website in download format.

Just type "Smema standards" in your search box and go


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Ken Bliss


SMEMA standards | 18 September, 2002

Hi Dilbert

I do not know if you meant to use the words argue in your posting, but assuming it was not sarcastic I would like to offer my point of view as an equipment supplier to people like you.

First the customer is not always right but they are never wrong, and no supplier should ever tell the customer they are not right, and certainly not argue with any customer, besides customers are hard to find these days, we need to keep everyone happy.

Second, they should be showing you before you bought it how it is set up to work correctly. When the customer does not care to read the instruction first that is the customers problem. I am guilty of this myself along with half the world. Regardless, the equipment supplier to be worth having around should always supply what information is needed for the customer to install and use the product successfully. If there is a standard that does not appear to match they should supply it to you.

Third, If you get an idiot on the phone, and we have all been there. My advice is to hang up and call back and ask for the service manager or higher as needed.

Fourth, If you are both supplying equipment to be used on the same line and working mutually together to sell to an end customer. Then management at both companies needs to reconnect with how to deal with these kinds of problems so the customers never see the problem.

In defense of the supplier and this is no excuse, there have been to many layoffs to adequately support customers, i know there are exceptions but the ones with problems seems to end up here at this forum like yourself.

I do not have the specific answer to the question, I know I am not the supplier and I think it was great the way you posted it without the suppliers name.


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SMEMA standards | 18 September, 2002

I started writing a long winded explanation about SMEMA and realised that it is just as easy to write about this as it is to talk about it on the phone. It just so happens that I have recently gone through this process so I believe that your vendor is right in that his downstream (next in line) is supplying you with a shortcircuit across pins 1 and 2 to indicate to you that his machine is ready to receive a board. A plain old shortcircuit should be easier to deal with than an opto output. You probably don't want an optocoupled input on your side because he's not supplying any voltage for you to be isolated from. This is how my usual conveyor vendor (maybe same) does it and I haven't had much problem with it. Maybe you are connecting to the wrong connector on his machine upstream, dowstream, next, previous... it's all very confusing. Talking about SMEMA interface is a little bit like dancing about architecture. The standard could have been written bit better or even proof read before being published - it has inconsistencies.

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SMEMA standards | 18 September, 2002

Here is how smema works: Machine A has a board to give to machine B. Machine A has an opto or a relay that will turn on or close when it is ready to pass this board to machine B. This is the "board available from previous" signal, telling machine B, "Hey, I have a board for you when your ready for it!".

How it works is inside machine B is an input with a pullup resistor on it. This signal is tied to the opto or the relay of machine A. When this opto or relay turns on or closes, it provides the signal back to machine B where it is tied to ground, and the signal goes low. So the "board available from previous" input on machine B goes low telling machine B there is a board available when it wants it.

Machine B tells machine A to send it the board with a copy of the same circuit except that machine B has the opto or the relay. This is the "machine ready from next" signal. Machine B thus finishes up what its doing with the board it has, gets rid of that board, and then looks at it's "board available from previous" input. Seeing that machine A has a board for it, it turns on the opto or toggles its relay thus returning the "machine ready from next" signal back to machine A telling machine A, "I'm ready for that board now, send it over!". Machine A then transfers the board to machine B, which turns off its opto or opens its relay to tell machine A that it has the board. Machine A then turns off it's opto or opens its relay to tell machine B that it no longer has a board to give to machine B.

Here is my problem: The opto has a diode across it to protect it and if machine A's wires are mixed up, machine A "board available from previous" signal will be at the anode of the diode which will always conduct and will pass the signal back to machine A where it will be pulled low. The result of this is the conveyor (machine A) will try and feed a board to machine B when machine B is not ready. Since the manufacturer of machine A uses relays the wires to his relay can be swapped as the relay will open and current will not flow regardless of direction.

My delema is the manufacturer of machine A is using a relay and just wants to get the product out the door to as many customers as possible. He doesn't care if his "board ready from previous signal" and return wires are mixed up because he has interperated SMEMA standard 1.2 to allow for this. He says that the manufacturer of machine B should do what he does and get rid of that opto and stick in a relay.

In respect for all the machine B manufacturers out there I want to prove him wrong. I have SMEMA Electrical Equipment Interface Standard 1.2 which on page 2 figure 1 vagely labels the signal lines. Can anyone out there say "Yes your right, stick to your guns and call him back.", or do I get the pin extraction tool and quietly swap the wires and say nothing?

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SMEMA standards | 19 September, 2002

Dilbert, You are both right! Just above the schematic in the Smema standard it says: "Conformance to the standard may be achieved by the appropriate design of the equipment or by providing special adaptors which enable the equipment to meet the standard."

So in fact this so-called standard is 2 standards; one using optos, one using relays, and they are not really compatible so you can't mix them.

The easiest solution is to supply an adaptor to convert opto output to relay output, or vice-versa.

It's sad that even in something as simple as an "on-off" signal the industry can't even settle for the simplest solution and instead propose 2 incompatible ways of sending the signal.

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