Month: February 2015

Categories
Process Control

Modbus Plus communication to your PC without a $2000 USB Dongle

I’m Jason Firth.

A few months back, I had a discussion with someone about how to communicate with a PC over Modbus Plus without breaking the bank. I decided to take the highlights of that conversation and bring them together in a blog post so others might be able to use the information.

Modbus Plus is still a widely used protocol in industry where Schneider Modicon PLCs are in use. However; it’s not a cheap protocol to work with. A USB Modbus Plus dongle for your PC will run you over $2000!

That’s a lot of money if you just want to look at some bits in the PLC. Today, I want to look at some other options.

Modbus Plus is a completely different protocol than Modbus.

Modbus Plus uses a proprietary signalling standard, where Modbus generally uses RS-232 or RS-485. Modbus Plus supports routing, where Modbus has no networking features. Modbus Plus requires a DSP to handle the communications, where Modbus can use a standard UART. Modbus Plus is peer-to-peer, where Modbus is master/slave. Modbus Plus uses a token passing system to ensure everyone gets a turn on the line, where Modbus doesn’t have any mechanisms (hence requiring the master/slave architecture where the master dictates who will speak). You just set your addresses, and all the devices will start talking. The Bridge Multiplexer acts as a gateway between the two protocols.

Modbus Plus allows for routing between devices called Bridge Pluses. The way you do this is by defining the modbus plus address of each bridge plus you pass through. For example, if you have node 1 on a modbus plus network with a bridge plus node 64, and there’s a node 32 on the other side you want to communicate with, you’d be communicating with 64.32.0.0.0

So to make sure your modbus devices can talk over a modbus plus network, you need to map the single values to modbus plus addresses.

Modbus plus requires a DSP. This means that no matter what, you’re going to need a dedicated piece of hardware to communicate on the network. You can’t just slap an RS-232 pigtail together and hope it will work.

Here are two potential options: first, if you get a PC with an ISA slot, the cards are quite inexpensive. I found some on eBay for 100-200 USD. A PICMG backplane can use an ISA slot, and there are industrial main boards still available. The downside to this is that you may be stuck using a very slow PC just to communicate with your one PLC.

Another option is a bridge multiplexer, which converts modbus plus to modbus. It takes a bit to put together, but it should work. I found a bridge MUX on eBay for 100usd when I first investigated this option. Today, I’ve found them for under $250 on eBay.

The manual really overcomplicates things.

There are models on page 10 of the manual — nw-bm85-000; NW-bm85C000; and NW-BM85D008 which don’t need a special progran. You don’t need to make a C++ program. You connect to one of the serial ports (I think the second one) and put the bridge into programming mode by flipping a DIP switch on the back, then it gives you a fairly nice and easy menu based interface to configure what it does.

You’ll need to figure out master/slave stuff, because modbus is client/host but modbus plus is peer to peer, but it should be very doable.

The master in a modbus interaction is the device that actually sends the commands. For example, I programmed a modbus TCP library, and in that case, the device that initiates the connection is the master: you connect, then either tell the device you want to read or write a coil or register, then the slave device that you connected to will respond with either a success/fail message for a write, or the data you wanted or an error message.

The Peer-to-peer feature is relevant because it means nothing on the Modbus Plus network is a master or a slave. They’ll just take care of communications on their own. In my tests, the only problem I could cause to the modbus plus network is if you set your bridge mux to the same address modbus plus address as something else on the modbus plus network. If you set a wrong modbus setting in the text interface, all you’re going to do is not communicate with the Modbus Plus network over the Modbus port.

Here’s how I managed to talk to a PLC using my PLC software and a BM85 connected to my serial port.

I set my modbus plus address of the BM85 to 2 by turning the first switch on and the rest off.

I entered config mode using the dip switch on the back

Once the screen loaded, I entered the following commands:

V1

P1

Tm

B9600

S1

Re

Mr

Y1

V2

E1 01 20 00 00 00 00

V4

W

Press y to confirm

Turn off bm85 and return config mode to run mode.

One key thing seems to be the rs-232 cable from the PC to the BM85. I used a 990NAA26320 but the wiring diagram should work ok to make a cable similar.

So what you’ll have is:

Your PLC, and bm85 daisy chained together on the modbus plus network.

Your PC plugged into the BM85 on modbus port 1.

Your BM85 set to a free Modbus Plus port

Your PLC set to whatever it is (no change)

Your bm85 configured using the keystrokes above, in run mode.

I use Fasttrak softworks, so I open it up and set it up to look at the COM port. Next, I did a PLC connect and did a port scan. I saw the BM85 and the PLC. Next, I chose the PLC from the list and hit connect. I was talking to the PLC!

So what’s going on: In this situation, you have two types of communication going on: The Modbus Plus communication, and the Modbus communication. So the PC acts as the master in a modbus communication. It sends commands to the bridge mux. The bridge mux, while configured with the master option, is actually acting as a slave on the modbus port: It is only responding to commands the PC master provides. The BM85 receives the message, and saves it so it can send a corresponding message on the modbus plus network to the PLC.

Now, you have another network, the Modbus Plus network. What’s going on there is, each device gets a token which is its turn to speak, and it says what it has to say on the modbus plus network and passes the token to the next device.

If the PLC and the BM85 are all configured, all this is transparent behind the scenes.

So let’s talk troubleshooting.

If you seem to have all the right pieces connected in the right way, I’d start from the outside and work my way in.

Can you connect to a PLC using your directly using the serial cable? If so, then you’ve proven the converter and the cable. I know that those serial converters can be flaky — If you plug them into different USB ports, they’ll use different COM addresses. You can check device manager to make sure you’re configured for the COM port you think you’re using.

What is your BM85 Modbus Plus LED doing? The following are the flash codes for Modbus Plus:

Six flashes/second Normal operating state. All nodes on a healthy network flash this pattern.

One flash/second The node is off-line. After being in this state for 5 seconds, the node attempts to go to its normal operating state.

Two flashes, then OFF for 2 seconds The node detects the network token being passed among other nodes, but it never receives the token.

Three flashes, then OFF for 1.7 seconds The node does not detect any token passing on the network.

Four flashes, then OFF for 1.4 seconds The node has detected another node using the same address.

If the light is flashing 6 flashes per second, then it suggests that your Modbus Plus network is working correctly.

Thanks for reading!

Categories
Management and supervision

Certified Engineering Technologist? Not here, you aren’t!

I’m Jason Firth.

There’s one thing most people don’t know about the law that they should: The law isn’t the same everywhere.

Often, people will talk about “how things are”, as if their experience in their location describes everyone’s. That’s incorrect, and it’s quite dangerous.

In an earlier entry, I talked about what it takes to become a Certified Engineering Technologist, and in another entry, I talked about what it takes to become a red seal Journeyman. I know first-hand about these things because I went through the process in 2013.

However; in 2013, I also made a mistake. I applied for, and achieved, my Certified Engineering Technologist designation in Manitoba. At the time, I didn’t know if I was covered nationwide, so I called the Canadian Council of Technicians and Technologists and asked if I could use my designation across the country. They told me it was fine.

They were not being entirely truthful. In 2010, the governments of British Columbia, Alberta, Saskatchewan, and Ontario split from the Canadian Council of Technicians and Technologists to create Technology Professionals Canada, a new organization dedicated to the profession of Engineering Technology in Canada.

As a result, and as a result of the wording of Section 11 of the Ontario Association of Certified Engineering Technicians and Technologists Act, 1998, S.o. 1998 C.Pr7, the use of the CET designation is restricted and it is an offense for anyone who is not a full member of OACETT to use the title.

Not realizing that the title didn’t automatically transfer like a red seal, I used my CET title in Ontario, only to receive a Cease and Desist letter from OACETT’s lawyers.

In my case, I asked about my options as a member of CTTAM, and the lawyer told me:

1. You can maintain your primary membership in Manitoba and apply to OACETT as an out-of-province member. You will pay full dues to Manitoba. You will need to pay out-of-province member’s dues in Ontario which are one-third of what a regular member pays;

2. You can transfer your membership to Ontario; or

3. You can transfer your membership to Ontario and maintain out-of-province status with Manitoba (assuming Manitoba has this provision).

After paying a small fee, I was able to transfer my membership to Ontario without any further difficulty. It took about a month, during which I stopped using my designation in Ontario.

I ended up taking the third option, transferring my primary membership to the province I practice in, and using an out-of-province membership (at a cost of about $100/yr) in Manitoba.

Something to keep in mind!

Thanks for reading!

Categories
Management and supervision

You need your own competence!

I’m Jason Firth.

So, now that oil has dropped dramatically, oil companies are discovering something I’ve always said: You need your own competent people.

The oil and gas industry has found out in a very abstract way: They’ve realized huge cost overruns on projects since they decided to let other companies monopolize engineering competence. When they had their own people working on the projects, they didn’t have the same cost overruns.

There’s a number of reasons why this is the case.

Let’s start off with what I like to call institutional memory. This is the effect of having people who have worked in your facilities for years making decisions about those same facilities. You can’t outsource institutional memory. It’s something that either belongs to you, or is lost. When you’re making decisions that affect the short-term viability of a project or the long-term viability of a facility, there’s a million tiny facts which you need to know, and you won’t learn in a 2-day site visit. Those folks don’t know about the fifteen other solutions that were tried on a problem until the sixteenth worked. They’re stuck asking why something is done a certain way. Sometimes this means they’ll be going back to the failed solution 1.

Then, there’s the pure question of “Who do you work for?” — An outside company will always be looking out for their own bottom line. If a conflict arises between the interests of your company and theirs, they are going to be more likely to act in their best interests.

Along those same lines, if you’re dealing with your own talent, then you can make sure you’ve got the best talent on the most important jobs. I’ve seen it in the past where an outside firm will wow a client with the A-team, then once the contract is signed, send in the Z-team.

Speaking of contracts: Of course everyone agrees that scope creep is dangerous. However; there’s a certain flexibility that comes with dealing with your own people that allows economies. For example, I was working on one big project with an engineering firm, and another different project was going on at the same time. The engineering firm refused to do anything that wasn’t part of the plan without negotiating a scope change and fee increase. Meanwhile, I plugged two cables in, successfully connecting the projects together, and changed a couple lines on a CAD drawing.

The formality of dealing with another company can also be a danger when you rely on other firms to have your competence. While working for a company, there’s been a number of times when I’ve been working on some skunkworks for a company alongside the main, official jobs. Skunkworks is a term that comes from Lockheed Martin’s Skunk Works development group, which developed the U-2, the SR-71 Blackbird, the F-117 Nighthawk, and the F-22 Raptor. The designation is widely used in business, engineering, and technical fields to describe a group within an organization given a high degree of autonomy and unhampered by bureaucracy, tasked with working on advanced or secret projects. The concept of that autonomy and relaxation of bureaucracy is incompatible with the formality of a business to business arrangement.

All of these problems reduce economies and prevent innovation within companies, and facilities. You can’t outsource your institutional memory to another company. Your employees will always be the most aligned to your interests. You are only in full control of your own employees. Contract work can be inflexible and cause large cost increases. You can’t give other companies autonomy the way you can give your employees autonomy.

Thanks for reading!

Categories
Process Control

What’s inside a Modbus Plus connector?

I’m Jason Firth.

Eventually, I knew I would be writing at length about Modbus Plus.

Modbus Plus is the protocol that Modicon created to supersede the Modbus protocol. It has some superficial similarities, but it quite different under the hood.

Modbus is a Master/Slave protocol. One device is the master, and tells which Modbus slave to talk. By contrast, Modbus Plus is a peer-to-peer protocol. Each Modbus Plus device can request data from any other device.

Modbus doesn’t have any real way to manage congestion, because there should never be congestion. The master requests data, and will not request more data until the first is sent. By contrast, Modbus Plus uses a token passing mechanism, where each node in the network gets a chance to talk, then once it has finished talking it will pass the token to the next node.

Modbus generally relies on RS-232, RS-422, or RS-485 signalling to communicate. By contrast, Modbus Plus uses a proprietary signalling protocol over a single twisted pair of wires. Modbus can be implemented using a standard UART, where Modbus Plus requires a special DSP.

Physically, Modbus Plus is a bus protocol. All devices are electrically connected to every other device on the Modbus Plus network through an electrically continuous shielded twisted pair cable.

Modbus basically doesn’t have network capability by itself. However, Modbus Plus has basic networking capability. Using a device called a “Bridge Plus”, you can connect different Modbus Plus networks together. The address you use to connect to a device is actually the path of devices you follow to get to the other device. If you were connecting to device 1 on the local Modbus Plus network, then you’d connect to 1.0.0.0.0. If you were connecting through a bridge multiplexer at address 2, then you’d connect to 2.1.0.0.0. If you went a step further and connected to 1 through yet another bridge multiplexer across the network at address 3, then you’d use address 2.3.1.0.0.

The data transferred using Modbus and Modbus Plus is roughly equivalent. You can send and receive inputs and coils, inputs and registers. There are also other operation codes in the protocol for diagnostics, or programming PLCs, or a number of other functions.

There are 3 types of connector routinely used for Modbus Plus. The AS-MBKT-085 inline connector takes a Modbus Plus cable and stabs the wires, to establish continuity. The 990NAD23000 Tap takes that cable and stabs the wire into a “tap”, which connects to a moulded cable which connects to the device. The 990NAD23020 or 990NAD23021 Supertap doesn’t stab the wire, instead using screw terminals to connect the Modbus Plus cable and the moulded drop cable.

Today I want to look at the AS-MBKT-085 inline Modbus Plus connector. We’re going to look inside one.

The first thing you need to realize about these connectors is, they’re not cheap. This store is selling one for 35 USD, and that’s about what you can expect to pay. I’ve seen some online stores asking for twice that.

So, what are you getting for your money? Well, first, let’s look at what this thing does.

Here, you can see a Modbus Plus connector with wires already crimped into it.

Normally, the shield grounds to the middle pad, and the wires are held in place by the plastic back, which is itself held to the connector with a screw.

The stabs which hold the wires are fastened to the connector with screws, so we’ll pull them out.

I expected the connector to be welded shut, but it’s actually held in place with 3 pegs which push into 3 holes. Once I had a point I could leverage, I could pry the connector apart.

So here’s what we have: 3 metal block with a threaded hole, connected to a regular D9 connector.

Taking apart the connector further, I found that the ground is coupled to the chassis ground using a capacitor. That’s all there is to it!

And how about those light grey plugs? The difference is that there’s a 120 Ohm resistor across the data pairs. That’s the difference.

Thanks for reading!