The criteria for choosing a solution for the PLC-to-1Wire communication were as follows:
- It should be quite cheap. My system works and any extension is more a caprice and an expression of interest than a response to a specific need. Solutions based on WIRE-CHIP sold by Solid Chip are well tested and functioning by other users. The require, however, the WIRE-CHIP (some EUR 150), RS485 communication module (WAGO 750-653, another EUR 150 on ebay.de) and… the knowledge of MODBUS protocol. I was looking for a system, which would manage the 1-wire network and respond to PLC queries send as POST/GET, meaning, which would work without extending the PLC itself,
- It should be quite simple. I do not know C, understand nothing of microprocessors, compiling system cores, etc. I need something, which is well described and supported by articles/blogs/forums available on the web,
- It should be quite standard to enable a quick exchange in case of a malfunction or an upgrade need.
- a rich list of other utilization options would also be desired.
On the other hand it was not critical to:
- Have a solution, which is very quick. I will be updating the temperatures at frequency above once-per-minute,
- Be as reliable as the PLC itself – as a potential problems will not cause an avalanche of consequences on the side of the PLC.
Hearing such a lest of needs my friend, who is a microprocessor specialist responded quite quickly: buy a Raspbery Pi (RPi).
RPi is a small (9x6cm) computer platform based on an efficient microchip and offering a great number of in/outs (2x USB, Ethernet, HDMI, SD i many 'pins'). It is also quite cheap – the final price amounted to c.a. EUR 40 (Feb. 2013).
The community around the RPi is quite extensive. There are many forums and blogs describing various projects.
RPi works under Linux, it is therefore an independent ‘PC’. One can even pay a full HD movie on it, what – obviously – is irrelevant from my current perspective, but shows the potential of the device.
A quick google search showed also that there are many projects describing how the 1-wire sensors can be connected to RPi. Such information increased the probability of success and convinced be to push the “Buy now” button. I ended up with the RPI, a 2A microUSB power unit and a 8G SD card – all for a total of EUR 60.
Here are links, which can be of use for those looking for additional information:
- The main page of the Raspberry Foundation with news and simple instructions (Quick Start) and precompiled operating system images.
- Adafruit web page where a few instructable of different RPi usages are presented – including the 1-wire sensors.