News » Modbus and back...

Modbus and back 8/12/08

Well, it's been a while since I updated anything on this site...

Having had a change of job a wee while back (The full time sort), and having some round the house domestic type projects I've not done a lot of fiddling in the shed, as it were.

But I have had an epiphany of sorts.

I started out the Myio project intending to make a 'sorta modbus' interface / protocol. I quickly realised I could just do the whole of modbus in a pic16F870, so off I set on a journey to make a modbus RTU.

Well, in the middle there, around about the last update I typed, I made the Parallax Basic Stamp 1 RTU.

Turns out it's been running non-stop since then, give or take some power cuts, and I've had a whole lot of fun with that and the 'sorta ladder logic' tool that I wrote for it.

So, having gained a bit of free time over the last couple of weeks I decided to get back to my modbus unit, which has led to some soul searching, and a change of direction to something halfway between the BS1 RTU and a full blown modbus RTU.

My reasoning can be shown thusly:

DIY Modbus RTU

Pros                  Cons
Standards based Complex coding exercise
RS485 interface Sledgehammer for my needs
Not 'all my own work' = not all that satisfying
Large(ish) micro required.


Pros                  Cons
Simple Blocking serial routine
All my own work! Slow
BS1's are cool, but not cheap
RS232 interface (short cable runs)

So, what I really want is to re-invent the wheel with an RS485 connected, simple RTU that is 100% of my own creation. Unfortunately that means the standards based bit goes out the door, but heck, this is a hobby, who needs standards anyway? I'm using RS485, it's a standard, that's enough.

On that basis, I'm back at the drawing board with the whole RTU project. I've learned a great deal from my modbus coding experiment, and working with the BS1 as an RTU, so the effort wasn't wasted by any means.

My new goal is a set of small modular RTUs that provide a small number of IO channels per device, using an RS485 interface and a somewhat simplified protocol. I'm thinking I'll use one of the 18 pin PICs, probably the 16F88.

I've got some ideas for some automation projects to use the as-yet un-designed, un-built RTUs, and I'm relatively clear in my mind as to how I'll go about building them.

Watch this space .. :-)

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