# RS232 vs EIA-422



## acro (May 3, 2011)

I am putting a DRO(Digital Read Out) on my 2nd milling machine and ran into a bit of an issue.

The new scales are not able to be read by my new DRO. The new DRO does read the scales on my other milling machine, so I believe there is no issue with it. My old DRO will not read the new scales either. I have tried various options in the setup screens but nothing seems to work.

Perhaps it's as simple as changing the pinout of the connector, but I am not sure. I purchased them back in February, and have contacted the seller of the scales, but no response yet.


Looking at the data from the ebay listings, I think the problem is the DRO is EIA-422 and the scales are RS232. Would it be as simple as just switching some wires in the connectors on the scales? Or would I need actual converters? Connectors are DB9

Thanks


The DRO is this one; item 192822714259

It shows the comm parameters for the DRO


> 1---Empty
> 2--0V
> 3--Empty
> 4--Shield
> ...


And these are the scales; item 164316357410 
They only describe the interface as Linear Scale Interface: DB9(RS232) which is;


> 1DCDData Carrier Detect2RXDReceive Data3TXDTransmit Data4DTRData Terminal Ready5GNDSignal Ground6DSRData Set Ready7RTSRequest to Send8CTSClear to Send9RIRing Indicator


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## emtnut (Mar 1, 2015)

Yes, you need a converter. Been long since I worked with that kind of stuff, but they were a dime a dozen back then.

I looked at the listing, and they mention rs-232, but then show the connections as 422 TTL ??? Unless it's switch selectable ?


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## mdforbis (May 19, 2011)

EIA-422 is similiar to RS-485 and you might have a better chance of finding a converter from RS-232 to RS-485 instead that would probably work. 422/485 is a differential signal where the difference between A and B defines a 1 or 0 instead of RS-232 which is an absolute voltage level signal, so they are not compatible without a converter to convert the voltage levels.


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## acro (May 3, 2011)

Well shoot. I don't think either device is selectable. 

So, something like a DTech DT 9003?


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## mdforbis (May 19, 2011)

acro said:


> Well shoot. I don't think either device is selectable.
> 
> So, something like a DTech DT 9003?


Yes, something like that should work based on what you have posted assuming it is just a different physical interface. There could also be protocol differences in the software which there is not enough information to know.


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## Lightsmith (Oct 8, 2010)

As mentioned with RS422 and RS485 there are two voltage states on the data line and this enables the signal to be sent over greater distances reliably. With RS232 the voltage can drop to where the device cannot see the "+" state. With RS422 the voltage difference between the two wires is used and so if both drop 30% over a distance the device is still able to see the voltage difference and determine the signal state. 

This is where going to a source for electronics for factory floor use is going to provide the adaper needed.


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## bill39 (Sep 4, 2009)

Black Box Electronics has reliable products and good support.

Look at some of these:





Converters | Black Box







www.blackbox.com


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## GrayHair (Jan 14, 2013)

We used a lot of B&B products including *converters*. Good prices, usually in stock and good support.


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## just the cowboy (Sep 4, 2013)

bill39 said:


> Black Box Electronics has reliable products and good support.
> 
> Look at some of these:
> 
> ...


Support is a real good point, that phone call can be priceless.
AD has this one that I have used and they offer good support also. https://www.automationdirect.com/ad...ssories/software_-a-_communications/fa-isocon
Cowboy


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## acro (May 3, 2011)

Well, I got the DT-9003 converters and have not had any luck getting my DRO working.

I believe that I may have neglected to consider something. The RS232 device does not have its own power supply. The RS-422 is what is powered. I believe that the converter needs the signal to be generated on the RS232 side.

While the devices from BB may be fine for some applications, the price point puts it out of the running for this application. The scales themselves are about half of the BB converter.

And the advantech and AD ones are about even money. I would need 3 of them and I guess my hail mary pass with the approx $10 option from ebay probably won't work unless i've failed to consider something.

Looking back at the published pinouts, the single difference on the DT9003 and the scale is pin 1. GND on the DT9003 and it says carrier detect on the scale info. However, the biggest difference is on the RS422 side.
From the DRO ------------------------------DT9003
1---Empty ---------------------------------Send A+
2--0V ------------------------------------- Send B-
3--Empty --------------------------------- Receive A+
4--Shield --------------------------------- Receive B-
5--Empty --------------------------------- GND
6--A ------------------------------------- VCC
7--+5V
8--B
9--R

To me, those appear to be incompatible.



Anyone have an idea to throw at me? Am I on track with my thinking?

Thanks


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## emtnut (Mar 1, 2015)

I took a look at the listing for the scale. Unless they changed the listing, it is also showing a 422 interface !?

From looking at both the listings you showed, it seems like they are both 422, and you just need to make a custom cable to match up the pins.


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## just the cowboy (Sep 4, 2013)

Try This:
DRO pin 2 to DT9003 GND
DRO pin 6 to DT9003 Send A & Rec A
DRO pin 7 to DT9003 VCC
DRO pin 8 to DT9003 Send B & Rec B

Cowboy


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## acro (May 3, 2011)

Thanks for the help fellas. I did get a response back from the seller, but it really wasn't much specific help but they did clarify the pinout of the connector.

I did overlook a photo that shows the DB9 pinout of the scales. Emtnut, you may be right - looks more like 422. Looking at the pinout, it doesn't look like RS232 that is in the description.

And Cowboy, I'm gonna hold up on that just a minute. Let me re-think this with the new pinout info. May not need the adapters.

DRO -------------------------------------Scales----------------------DT9003
1---Empty --------------------------------+5V --------------------------Send A+
2--0V --------------------------------------- 0V -------------------------Send B-
3--Empty ------------------------------------A --------------------------Receive A+
4--Shield ----------------------------------- B -------------------------- Receive B-
5--Empty ---------------------------------- NC --------------------------GND
6--A ---------------------------------------- NC ------------------------- VCC
7--+5V ------------------------------------- NC
8--B --------------------------------------- NC
9--R --------------------------------------- NC

So, maybe I just need to re-order the pins in the scale connector like emtnut says.
Move..
1 to 7
3 to 6
4 to 8

Simple as that? No converter needed?
probably give it a shot tomorrow after work.


Thanks again.


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## emtnut (Mar 1, 2015)

acro said:


> DRO -------------------------------------Scales----------------------DT9003
> 1---Empty --------------------------------+5V --------------------------Send A+
> 2--0V --------------------------------------- 0V -------------------------Send B-
> 3--Empty ------------------------------------A --------------------------Receive A+
> ...


That looks good. Just to note, you still have the DT9003 in your diagram, it's not needed (I think you already know that thou).
Hope that gets it going !


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## acro (May 3, 2011)

Sorry, got to wait another day. Got a last minute request to be a sub in golf league. 😅


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## splatz (May 23, 2015)

I may be wrong but I don't think there's a pinout / crossover / null modem cable that will convert RS-422 to RS-232. The signalling is different, different voltages, etc. With two RS232 devices if you connect transmit to receive, receive to transmit, and signal ground to signal ground, it can usually work. But if you connect an RS232 transmit to an RS422 receive-plus, the RS422 will be looking for voltage in reference to the receive-minus. If you connect RS232 signal ground to the RS422 receive-minus, the RS422 will see a much higher voltage than expected. In the other direction, the RS232 will not pay any attention to the much lower RS422 voltages, it will discard them as noise. 

But I like I say I have had serial comms I thought should not work work, so I might be wrong.


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## emtnut (Mar 1, 2015)

splatz said:


> I may be wrong but I don't think there's a pinout / crossover / null modem cable that will convert RS-422 to RS-232.


True. RS232 is a +- 12V unbalanced signal, and 422 is +- 3V differential signal ... Needs a converter.

It seems the scale and DRO both are 422, just different pin out assignments. (mix up in the description)


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## acro (May 3, 2011)

Success!

No adapter needed. 
Thanks all.


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## GrayHair (Jan 14, 2013)

acro said:


> Success!
> 
> No adapter needed.
> Thanks all.


Seems strange, but you can't knock success.
Had to convert 232 to 422 for long runs at one place. Security chief wouldn't allow anything security related on the network. No idea why 422 was chosen.


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## emtnut (Mar 1, 2015)

GrayHair said:


> Seems strange, but you can't knock success.
> Had to convert 232 to 422 for long runs at one place. Security chief wouldn't allow anything security related on the network. No idea why 422 was chosen.


Distance is limited to about 100' with RS-232. RS-422 is about 4000'. RS-485 is also ~4000' which is what the old A-B Datahighway+ was.
The other thing is 422 has much better noise immunity .


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