PIC Micro Modbus Update 1 : Breadboard mock-up

I was sitting in my workshop looking for something other than painting/staining… I have project overload for sure, so I was looking for something that was 180 degrees out from what I had been working on. On my whiteboard I have one of those “been-on-the-TODO-list-for-at-least two-years” projects aptly named “MODbus”.. After determining that a BACnet controller wasn’t going to run on a 8-bit micro (many moons ago) I figured I’d try it’s much lighter weight cousin MODbus. It was time.

MODbus is a older-than-dirt communications protocol for industrial process/power controls. It’s generally seen in the wild as MODbus/RTU which is a RS-485 protocol ran at 9600 or 19200 baud. Of course there are a ton of variations and it wouldn’t be a comm protocol these days unless someone decided to make a MODbus/IP.. but that’s beyond the scope. Google, as always, is your friend. Super basic protocol the master polls the loop with the address, a basic command, and what data points it’s affecting. The slave responds with it’s address, the command the resulting data as required. The error correction is CRC 16bits.. never used CRC before.. seen it a million times, so this will be fun. In the beginning I’ll just ignore it and send it some pre-determined correct data as needed.


Step 1.. pick a micro, because I don’t feel like making process flow charts just yet.

I might try Jason’s new dev-board that’s on the way now, hopefully it’ll pop in tomorrow or next week. For now I’m working on the 16F1509. I don’t think it’ll stay on this just because I feel like mixing it up a bit. I’ll switch it to the ’14K22 at the very least, the ‘1509 was in the board already and wanted to get going on this project. Note: I didn’t even successfully complete Step 1. 😀


Step 2. Determine what’s a good target for a successful test?

Well I like to break dev up into little pieces.. so I have as simple of goals as possible:

2a. I just wanted to prove to myself my 8x DIP switch was read to choose a slave device address. Check, no brainer.. that was easy. CPU timing seemed off through… oops:! 0x0D is 4MHZ.. I want 0x0E! (8).

2b. I got the serial going, okay… sending 0x2E (“.”).

2c. I found a MODbus master polling simulator that will work for the next 29 days (it’s 198x tech people, why are we charging for the basic tools?) .. I’ll build one of those later… and now..

2d. Have the PIC recognize that it’s being polled. FAIL!


That’s where I ended off for the evening. Scratching my head why my received address isn’t recognized (when it does it should output a pulse on a known good output).. the LA didn’t pick up anything but it is going through interrupt for the RX. I guess it’ll be a nice Saturday morning project with some tea.


The logic analyzer ascreenshot. I am getting good packets.. just not recognizing them yet.
The logic analyzer ascreenshot. I am getting good packets.. just not recognizing them yet.

In the screenshot I didn’t rename the channels, so Channel 1 is RX (from the PC running the sim), TX is channel 3. I’m dumping the RX buffer into a temp variable and transmitting it when it makes it through my main loop.

No sense in sharing the code just yet.. it’s a hacked up pile of festering dog crap. The breadboard even looks a little chaotic!


Mock-up of my MODbus controller breadboard style version 0.00001.
Mock-up of my MODbus controller breadboard style version 0.00001.

So why MODbus? Why not! I have four reasons for taking up this project, now more than ever.

1. I would like to implement a small comm system in our new home.. MODbus is light weight enough to do it on 8bit micros, an I can use something more powerful for control if I want to later.

2. At work our BACnet supervisors (master units) are getting native MODbus drivers in the next sub-version (instead of using integrators); that’s cool, I want to test it out.

3. I want to turn the whole thing into an article of 0xEE.net after I’m done… bonus.

4. why not? I’d like to experience.. plus I already told co-workers I would so no way I can fail now!


  • SOI Sentinel
    December 11, 2014 - 10:35 am | Permalink

    Modbus/TCP is alive and well in the industrial Ethernet world, in fact Schneider Electric still uses it as a primary communications protocol, and almost every other vendor who deals in Ethernet connectivity has a ModbusTCP driver as it’s effectively open source, license free, and fairly lightweight. It’s just not natively deterministic. It’s also predisposed to allow the TCP module to act as a gateway to serially connected ModbusRTU units.

    • Chas
      December 11, 2014 - 3:09 pm | Permalink

      Thanks for the feedback! In my segment of the automation business MODbus is taking the slow death into obscurity. I still see it on almost all power and generation equipment as well as many IP-based sensors such as liquid flow meters. Everyone seems to be switching to BACnet.. at least they’re not opting for LON. I don’t think MODbus is going anywhere anytime soon though.

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