Vorticon's Blog - Wireless weather station managed by the TI 99/4A - La

RSS Bot · March 14, 2017

So it's time for a new hardware interfacing project for the TI 99/4A, namely a wireless weather station managed by that venerable computer. I think it will be a fun project but likely with a steep learning curve. The goal is to have a functional prototype in time to demo at the 2017 Chicago Fair.

So where does one start? The most obvious would be finding a cost efficient and relatively easy way to achieve wireless communication between the various sensors and the computer. I believe the Xbee series 1 modules fit the bill quite nicely because they have a range of about 90 meters (300 feet) which is more than enough for the average user and they come pre-configured for serial communication. I chose the version that has a connector for an external antenna in order to maximize the transmission range.

These modules have integrated ADC as well as digital inputs and transmit data serially which can be captured by a sister Xbee module connected to the computer. Unfortunately, they would not be enough by themselves because there are few sensors that can interface directly with them without some sort of processing first, and so a microcontroller is going to be needed for support and data packaging within the sensor array. With so much community support and experience available for the Arduino, I decided to go with the Arduino Uno using the Sparkfun Redboard.

It comes with installed headers and connectors, making it easier to access and interface.

Now for the sensors:

While I could design and build my own mechanical components for wind speed, direction and rain level, it would be a pretty laborious process. So I opted to go with a ready made solution that is super easy to interface, again from Sparkfun:

This is a purely mechanical device with no embedded electronics, and the data interpretation is via checking the state of various switches. What is even better is that it has a direct interface with the Sparkfun weather shield for the Arduino Uno which also includes pressure, humidity and luminosity sensors.

All we need now is a temperature sensor, and the one below is inexpensive and pretty easy to use. It is analog, so it will be earmarked for one of the Xbee ADC inputs.

This should round up the sensors for our weather station. The last thing we need is the Xbee serial explorer board to connect a receiver Xbee module to the TI 99/4A via the serial port.

So far, this is going to cost around $215, and will likely need another $25 for various connectors and support items. So no, it's not exactly cheap, but the fun factors of putting all this together will be priceless!

Now what? Well, first of all I'm going to have to get familiar with the Arduino since I have had zero previous exposure to it. I have always used the Raspberry Pi for my other projects, but it would not have been suitable for this one. That is going to take a little time. But even before that, I'm going to have to dig deep into the Xbee documentation and come up with some communication tests, likely using only the temperature sensor for starters since it can connect directly to it, and see if I can transmit data wirelessly to the TI.

Update 1/28/17
First things first: Since the Xbee modules communicate via serial protocol, I needed to understand how that worked on the TI. It's definitively more involved than parallel communication, with a multitude of registers and CRU bits needed to be accessed in the 9902 UART chip in the RS232 card. Here Thierry Nouspikel's encyclopedic site ( http://www.unige.ch/medecine/nouspikel/ti99/titechpages.htm ) came in to the rescue with clear explanations and code examples.
Obviously this is all done in assembly which is fine. However, I recently found out the Rich Extended Basic (RXB) by Richard Gilbertson actually has a command called CALL IO which allows direct access to the CRU without resorting to assembly language! Below is an explanation by Lee Stewart about how it works:

CALL IO(type#,bits,cru-base/2,var,var)

If you set a variable named CRU to half the PIO CRU-base-address of >1300/2 = >0980 (2432), you can address CRU bits by adding to CRU in the call.

The following code will set, test and reset (clear) the indicated bits:

100 CRU = 2432
110 CALL IO(3,1,CRU,1) ' SBO 0 ;set bit 0
120 CALL IO(3,1,CRU+7,1) ' SBO 7 ;set bit 7
130 CALL IO(2,1,CRU+2,X) ' TB 2 ;test bit 2 and pass value to X
...
300 CALL IO(3,1,CRU,0) ' SBZ 0 ;reset bit 0

What this means is that at a minimum I could test my setup directly from within RXB without the need for coding in assembly with its associated cumbersome and time consuming development cycle of Edit/Compile/Run. And if it works out well, then there is a very good possibility that I might be able to code the entire project in RXB and bypass assembly altogether.

Update 1/31/17

At this stage I have configured 2 XBee modules for 9600bps 8N1 communication protocol, and attached one to an Arduino Uno via a shield, and the other to the serial port of the TI using an XBee serial adapter. The way XBee works is that if one of the modules receives data on its Data In line, it will serialize it and transmit it wirelessly to the Data Out line of all the other XBee modules in the same network. I have therefore connected the XBee DOUT line to the Arduino's Rx (receive) line, and the XBee DIN line to the Tx (transmit) line. There is a very small test program running on the Arduino which basically watches for data to show up on the Rx line, reads it, and if it is an 'a', it turns an LED on, and if it is a 'b', it turns that LED off.

On the TI side, I am running TELCO with the same communication parameters as the XBees, using a null modem serial cable. When I type 'a' in TELCO's terminal, the LED on Arduino lights up, and it turns off when I type in 'b'. Simple, but a great proof of concept nonetheless. Here's a quick video of the demo:

Update 2/27/17

On this next step, I switched over to using RXB on the TI side using the CALL IO functionality, and it worked beautifully, thus obviating the need to use assembly for this project. It took some fiddling and lots of help from the AtariAge TI community, particularly Stuart, to properly configure the TMS9901 chip in the RS232 card to work with the Xbee given that we are only using the Tx and Rx lines with no flow control whatsoever, but we eventually figured it out. The only downside of this kind of set up is that the Arduino is much faster than RXB, and so delays had to be introduced in order to get a reliable transmission, in the order of 500 msecs per byte transmitted.

From there, I moved on to actually connecting a temperature sensor to the remote arduino's analog inputs and tried to read the temperature data on the TI. The raw analog input from the sensor was converted by the Arduino to a float temperature value, which in turn was converted to a string and sent byte by byte to the TI. On the other end, the TI reassembled the string and converted the final product back to a float value. Below is the video demonstrating that experiment:

Update 3/13/17

So I've decided to add a real time clock to the project with the idea of logging the various weather parameters for future analysis and comparison. To that effect, I acquired the Sparkfun RTC module based on the DS1307 chip and communicates to the microcontroller via the I²C (inter-inter chip) protocol. Sparkfun provides a specific library for the Arduino that makes access to the various date/time parameters trivial.

Here's a demo of the process. Sorry about the quality of the video but I only had a low end camera on hand...

Stay tuned