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Offline SScnc

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ModIO Expansion Limit
« on: November 23, 2010, 11:19:54 AM »
I'm getting into the world of PLC's for my current job and was recently turned on to Homann Designs ModIO board for Mach3. I've watched the videos on setting up the ModIO and I have to say I'm really impressed with the amount of versatility you can achieve with this setup.

I'm left with two questions though.

I'm assuming since I haven't seen it mentioned anywhere, you can't use ModIO to send step/dir signals to stepper/servo drives. Is this correct? If so why not?

What is the limit on expansion ModIO boards?

Re: ModIO Expansion Limit
« Reply #1 on: November 23, 2010, 07:04:26 PM »

To answer the easy question first.

Each ModIO has a RS232 to 485 converter on board.
The ModIO can communicate via RS232 or RS485.
So, You can have one ModIO connected to the PC and then have all other ModIO's daisy chained to the first via the RS485 port. The theoretical limit is 240. The RS485 drivers on the ModIO will support up to 32 ModIO devices on a bus.

Have a look at the example in the link below.It shows this setup.


Using th new Modbus interface in Mach3 (Serial Plugin Support) you can setup 256 separate Modbus transactions. These can be to one or more ModIOs.

You then use Mach3 brains to interface between Mach3 internals and the Modbus data.

The Modbus Protocol is a very robust industrial strength interface. That said it has it's limitations. There are delays between Mach3 sending commands and getting back the replies. Therefore, you cannot use it for time critical motion control.  Home switches are also not suitable as there is an indeterminate delay (0-20ms) from when the switch is activated and when Mach3 receives the data.

To do Motion control, the ModIO would need to have a trajectory engine in it and take high level motion commands and then perform them.


Homann Designs
email: peter at homanndesigns.com

Offline SScnc

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Re: ModIO Expansion Limit
« Reply #2 on: November 24, 2010, 06:24:16 AM »
Awesome, thanks for the answer.

So pretty much anything but step/dir signals for drivers, and home/limit switches can be connected. You'll be seeing an order from me in the near future.

Thanks for making a great product.