DH485 and Powerflex

sparkie · Nov 24, 2014

Hey, I'm a student in a PLC class, and also a Control System tech. This is my first post. What I'm curious about is the DH485 connections on an SLC 5/03 firmware 301.

I have a Powerflex4 in class and for my project want to connect it to my slc 5/03 but we don't have any kind of DH485 module. If I am only connecting the 5/03 to the powerflex, is it possible to make that connection without a module? I can make whatever kind of cable/connection I need.

I also read in the manual that I only need pins 4 and 5 on the RS485 cable to connect between the two devices.

Ken Roach · Nov 24, 2014

Welcome to the Forum !

This is an excellent teaching moment to illustrate the difference between a serial signaling technology and a serial protocol.

The Data Highway 485 port on the SLC-5/03 controller does use RS-485 signaling. The voltage levels and signal biasing and cable types are all in accordance with the RS-485 standard.

But there are dozens, maybe hundreds, of different protocols that can run over RS-485 signaling.

The Powerflex drive uses one of those: its called "DSI", Drive Serial Interface. It includes several commands and data types in common with the granddaddy of industrial serial protocols, Modbus RTU.

Data Highway 485 is proprietary serial protocol developed and maintained by A-B for the SLC family of controllers in the 1980s.

While DH485 and Modbus RTU can both run on RS-232 or RS-485 wiring, they are different and incompatible protocols.

Ken Roach · Nov 24, 2014

You won't be able to connect these exact devices together.

If you were able to upgrade the SLC-5/03 controller to OS302 Series C FRN 10 or 11, you would get all the modern goodies of the SLC-500 firmware, including Modbus RTU Master functionality.

Then you would be able to connect the SLC-5/03's RS-232 serial port to a RS-232/485 converter and send Modbus RTU messages to the PowerFlex drive. I've done this with the A-B 1761-NET-AIC converter, as well as with various third party devices.

But if you're stuck with an OS301 controller, it won't support Modbus RTU Master functionality. And there's no practical way to connect the DH485 port to the PowerFlex.

If you really wanted to impress the instructor, you could fake Modbus RTU with the serial port of the SLC in ASCII mode for a handful of messages. You would still need an RS-232 to RS-485 converter. And I don't think that "do in the lab what you would never do in the field" is the lesson we're after.

sparkie · Nov 24, 2014

So what I'm gathering here, is that I need a cable: rs232 with a serial connector to rj45 Jack dh 485 cable, then I can upgrade my firmware and use the cpu as a master device. How do I go about upgrading the firmware? Can I do this for free? I own the slc so I can do what I want with it.

My backup plan is to use my analog card, but I have lots of networked drives at work, as well as networked plcs, so exploring this facet of the work is very important to me. I also have a background in networking, so I understand numbering systems and protocols, just so you guys know.

Ken Roach · Nov 24, 2014

Yes, you will need an RS-232 to RS-485 converter. I like the 1761-NET-AIC, but there are certainly others on the market.

You will need to upgrade the SLC-5/03 firmware. This is an EEPROM plug that has to be installed, then allowed to perform the firmware upgrade, then removed. It's expensive, but if you make friends with your local A-B distributor or sales office they might do the upgrade for you.

Using Modbus RTU protocol and MSG instructions in the controller is actually a method I advocate strongly against. SLC-500 controller network ports aren't made to do cyclic I/O control like network scanner modules are.

But it does work, and it's a good introduction into all the advantages and disadvantages of serial networking and Modbus RTU protocol.

rootboy · Nov 24, 2014

Ken Roach said:
And I don't think that "do in the lab what you would never do in the field" is the lesson we're after.

The dirt burner that I re-engineered as a mobile unit in California had its very own "hand rolled" Modbus ASCII slave routine.

When I chose the AutomationDirect Click PLC for the project, the first criteria was that it was cheap, the second criteria was that it was cheap, and the third criteria was that it supported Modbus ASCII.

Well, it doesn't. It'll do Modbus RTU or ASCII. As in plain old ASCII. Never mind that the brochure had this to say about its capabilities:

"The other ports support either Modbus RTU or ASCII protocol."

I see that AD still hasn't clarified this. Whatever.

After writing my own Modbus ASCII routine for the PC side (did I mention that the company that I worked for was cheap?), I was d***ed if I was going to scrap it because either I or AutomationDirect were a bit fuzzy on the finer points of English grammar.

The AD support engineer was neither the most helpful, nor even the most polite support guy that I've run into (FYI, as a support guy, being unhelpful and borderline rude is probably a bad combination), and told me that I was out of luck and maybe it was time to find a Modbus RTU program to do my data collection with.

Nope, I don't think so. So I wrote enough of a Modbus ASCII implementation in the PLC to get my temperatures and whatnot out to the data collection PC.

So careful with the "never do in the field" part.

ASF · Nov 24, 2014

And yet right underneath that, your signature reads "Just because you can doesn't mean that you should..."

rootboy · Nov 24, 2014

ASF said:
And yet right underneath that, your signature reads "Just because you can doesn't mean that you should..."

I post it as a reminder to myself... LOL!

I was wondering how long before someone would pick up on that. ;>

Ken Roach · Nov 24, 2014

I once had an analytical instrument that ran Modbus RTU Slave protocol with one important difference; they swapped the order of the CRC bytes.

No commercial logic controller or software product I've ever seen does that, so I ended up adapting the ControlLogix Modbus RTU Master sample code to use a 1734-232ASC module.

I don't recommend it.

rootboy · Nov 24, 2014

Ken Roach said:
I once had an analytical instrument that ran Modbus RTU Slave protocol with one important difference; they swapped the order of the CRC bytes.

No commercial logic controller or software product I've ever seen does that, so I ended up adapting the ControlLogix Modbus RTU Master sample code to use a 1734-232ASC module.

I don't recommend it.

Swapping the order of the CRC nibbles is pretty much standard practice AFAIK (as standard as you get with a standard like Modbus, that is), I suppose it's an endian thing.

It's more likely that they didn't swap the CRC bytes.

I've written a CRC/LRC calculator for Modbus that includes code for swapping the CRC. Here's my code for the CRC byte swapping:

Code:

MBStr = Hex$(tCRC)                                'The result needs to be byte-swapped (little endian to big endian) for the Modbus world
MBStr = Right$(MBStr, 2) + Left$(MBStr, 2)        'This gets the string swapped
CRC = Val("&h" + MBStr)                           'And this gets you the actual value.

So there is something to be said for rollin-yer-own, I suppose.

When I worked for Schneider, getting things to play nice on Modbus was some of the most fun that I had. Kept me busy at times...

(Should anyone like the entire source, including the CRC/LRC calculations, and any active/semi-dormant boogs that go along with it, you are more than welcome to it. PM me, or I can post it here on site. It was written in PowerBasic/FireFly, but it's pretty simple).

sparkie · Nov 25, 2014

Well, I suppose I'll talk to our automation guys at work if I ever get a chance and ask around to see if we have any eeproms. It is doubtful anyone in my shop even knows what those are. I guess for now I'll just use the analog output card to control the VFD. I really need to start experimenting with networking and PLCs, but there are some other things I can delve into before I start messing with that stuff, which will obviously require me to spend more money (as I'm pretty much having to buy old PLC hardware and learn this material as I go). There is no way my job would invest in any training for me, heh.

sparkie · Nov 25, 2014

Ken Roach said:
Yes, you will need an RS-232 to RS-485 converter. I like the 1761-NET-AIC, but there are certainly others on the market.

You will need to upgrade the SLC-5/03 firmware. This is an EEPROM plug that has to be installed, then allowed to perform the firmware upgrade, then removed. It's expensive, but if you make friends with your local A-B distributor or sales office they might do the upgrade for you.

Using Modbus RTU protocol and MSG instructions in the controller is actually a method I advocate strongly against. SLC-500 controller network ports aren't made to do cyclic I/O control like network scanner modules are.

But it does work, and it's a good introduction into all the advantages and disadvantages of serial networking and Modbus RTU protocol.

Okay, so as a follow up Ken, You mentioned I would still need a converter. Now this converts a serial connection. I'm wanting to use an RJ-45 jack with an ethernet cable as a connection. That module can't utilize an ethernet cable that I'm aware of. That is leading me to a bit of confusion.

rootboy · Nov 27, 2014

sparkie said:
Okay, so as a follow up Ken, You mentioned I would still need a converter. Now this converts a serial connection. I'm wanting to use an RJ-45 jack with an ethernet cable as a connection. That module can't utilize an ethernet cable that I'm aware of. That is leading me to a bit of confusion.

Does it look like this? (the D2-DSCBL)

http://www.citect-webhelp.schneider-electric.com/citect/Drivers/Subsystems/KSEQ/Content/dlse3d7x.htm

As an aside, kinda odd finding the pinout of a Koyo PLC on a Schneider site...

DH485 and Powerflex

sparkie

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