modbus

Does anyone have any information for writing a visual basic driver for modbus. I am trying to communicate with a Versamax plc, I have been using Kepware OPC but seem to have problems. I wanted to give Modbus a try.

Google

The manual GFK-0582D, GE Fanuc Serial Communications Manual, available here http://www.gefanuc.com/support/plc/m-s9030.htm has information on the Modbus commands and also on calculating the necessary CRC-16 checksum.

recently worked on a job using vb to talk to a servo via modbus
the guys at win-tech were very helpful. here'e a link
Here

Your biggest issue will be the calculation of the CRC16 used by Modbus. Take a look at the enclosed document,

Get the Modbus Specification

Basic is not the best language for writting drivers. I would use MSVC and write a .DLL that VB can access.

Has anyone done this in basic? If so, did your driver conform to the timing specifications. If so, how did you get your driver to conform to the timing specifications?

When I write a Modbus driver I have access to hardware timers that can accurately time the intervals between characters. I write the drivers in C with some assembly language.

The CRC is easy as the listing is in the specification. Meeting the timing requirements is hard.

If your Kepware driver is running on a PC that is too busy to send the characters out one after the other with out gaps or respond to messages promptly then, the problem is with the PC and not the driver. Writing your own driver will be frustrating because you will have the same problems with CPU time.

Use the look up table version of the CRC calculations in the specification.

chrisatwan said:

I am trying to communicate with a Versamax plc, I have been using Kepware OPC but seem to have problems.

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What driver in kepware were you trying to use to connect to the Versamax PLC? The reason I ask is I don't see a driver in Kepware for the Versamax line only 90-30's.

?

glaverty said:

What driver in kepware were you trying to use to connect to the Versamax PLC? The reason I ask is I don't see a driver in Kepware for the Versamax line only 90-30's.

Click to expand...

What difference should it make if it follows the specification?

It shouldn't but...

Peter Nachtwey said:

What difference should it make if it follows the specification?

Click to expand...

Peter,

You wouldn't think it would matter but I had a problem trying to connect to a CPU363 in an older version of kepserver. I selected the driver for a 360 CPU.

The driver list is:

90-30 - 311
90-30 - 313
90-30 - 331
90-30 - 341
90-30 - 350
90-30 - 360

I talked to one of there tech guys and he said it would be fixed in the next revision. Once it came out I pulled up the list. The CPU363 still wasn't in there but when I selected 360 it worked this time. I don't know why they would need to be CPU specific because they should all be talking GE SNP or GE Ethernet which ever driver you are using.

So I agree it shouldn't matter but it seems to.

I have a project coming up where I want to connect to a Versamax Micro with a serial to Ethernet adapter. I'm hoping it will work. If I can get the parts together I'll test it out.

Is the horse dead yet?

It is easy to write a Modbus driver if you have control over both sides of the communication link. One can relax the specifications on the timing and make the system work reliably. When making a product one must follow the specifications if it is to work with all other products. Now you have me wondering if it is really a Kepware problem or a GE 9030 problem. I think it is safe to say that at least one of those GE 9030 Models does not properly support the Modbus specification if it takes a different driver from Kepware to get each model of 9030 to work. It looks like Kepware has had to compensate for the differences between the GE 9030 models. Now I must ask "will my product work with all those different GE 9030 models?"

[RANT]
I hate calls from custumers when they blame me or my product when it is the other companies non-compliance that is the real problem. This is why Gregs comments have set off my alarms again. I have been in some pretty good pi$$ing contest with well known companies. That is why I say get and follow the specifications! Don't blame the small company just because they are small, blame the company that didn't follow the specifications.[/RANT]

So now it is back to the original question problem. Even if one does write a Modbus driver that follows the specification, does GE's Versamax PLC support Modbus properly?

In the end I think Greg has the right answer and buy the right driver for your model of GE.

I already use the kepware for ethernet communication the with horner ocs but wanted to try to get rid of kepware and try using a versamax plc, but today I saw some other types of opc servers and now wonder how hard it would be to write a ActiveX for tcp/ip communication.

Hey what do you know it works.

Ok I got all the stuff together and set it up, heres what I was using.

Versamax Micro PLC (IC200UAL006-CC)
Versamax serial to Ethernet Adapter (IC200SET001)
Kepware OPC Server V4.81.217 U

First of all to clarify what was said earlier, the Driver that I am using is GE Ethernet. When your setting up the Device it asks you for the specific model ie 90-30 - 350 etc.

As I stated earlier there is no Versamax PLC listed under Devices and I have had problems in the past with some other ones. When doing my test I tried different devices from the 311 CPU to the 360 and they all worked.

So as Peter stated the device doesn't seem to matter as long as you are using the right Driver.

Chrisatwan didn't state which Driver he was using when trying to connect with Kepware so I don't know what he was trying he just wanted to try Modbus.

At any rate you can connect a Versamax PLC to the OPC server using GE Ethernet.