Nuts and Volts - April 2014

we can set each one equal to any character or string that we want. Here, we just redefine sep. This example may seem silly, but it clearly demonstrates what happens when we include an optional definition for the sep argument.

It also should give you an idea of the formatting that’s possible in Python3 printing. (We haven’t yet solved our degree symbol problem, but we will before we’re done!)

5. Here’s another attempt to remove the space between the temperature and the degree symbol. As you can see, it also doesn’t work!

6. This one does the job. We just needed to manually include spaces where we want them to appear.

7. Sometimes, a print() function is too long to conveniently type on one line. Here, we solve that problem by defining the end argument as the empty string. This example also demonstrates how we can switch between single quotes and double quotes, so that we can print an apostrophe in the text.

8. I mentioned earlier that I like two different Python solutions to our degree symbol problem. The second approach involves something called “string concatenation.” In case you’re not familiar with that term, it’s simply a way of combining two or more strings (or characters) to make a single longer string.

In Python, the “+” symbol is used for two different purposes: addition and concatenation. If we place a + symbol between two numbers, Python adds them (duh!), but if we place the same symbol between two strings (or a string and a character, or two characters), Python concatenates them. The examples in this section are somewhat trivial, but they do demonstrate how concatenation works.

9. Here, we use concatenation to solve our degree symbol problem.

Note that we have to convert the two temps from numbers to strings for this to work. Also, note how we can define end as the empty string to get everything to print on one line.

Now that we’ve covered a few details of the Python3 print() function, we can move on to our remaining experiments. As you encounter the various print() functions in the upcoming Python programs, you may want to refer back to the above examples for clarification. Better yet, you may want to try a few of your own!

Experiment 2: Simple Serial Transmission from PICAXE to Pi

If you look again at the listing for the temp9700test.bas program, you can see that a significant portion of the code is devoted to the conversion of the raw data; first to Centigrade, and then to Fahrenheit. The conversion code is the most complicated portion of the program, and it results from the PICAXE’s inability to perform computations on negative integers, fractions, or decimal numbers. Fortunately, we’re about to get some mathematical help.

Since the PICAXE is helping the Pi to deal with analog inputs, the Pi is going to return the favor, and help the PICAXE with the math! In other words, we’re going to send just the raw data to the Pi and let it do the math.

First, take a look at the listing for the PICAXE program for Experiment 2 (tempSerial ToPi.bas). As you can see, it’s much simpler than the program we used in Experiment 1. The subroutines have been completely eliminated; the main do/loop is less complicated; and there are fewer variables.

However, there is one small complication. We need to include two new variables: hiByte and loByte. This is necessary because all serial communication is byte-based. In other words, we can’t simply send the ADCval to the Pi because that’s a16-bit word value; we need to send the high byte and low byte of that value separately.

Since w0 is comprised of b1 (high byte) and b0 (low byte), we’ve defined suitable names for the two bytes so that we can serially transmit them to the Pi.

Also, note that it may look like we’re sending a single 16-bit value in the sertxd statement, but the inclusion of the “#” symbol in a sertxd or serout statement actually results in a 16-bit value being transmitted as 16 separate bytes. We could have used the same approach, but it’s much easier to just send two bytes and let the Pi reconstruct the 16-bit value of ADCval.

Now, let’s turn our attention to tempSerialFromAx.py (which is the Pi program for Experiment 2) and discuss the try portion of the main while loop. In the program listing, some of the program comments are numbered. The following comments elaborate on the corresponding program comments:

1. As we discussed in the previous Primer, the pySerial read() function reads a single byte, and it’s a blocking function by default which means the program will remain at that point until a serial data byte is received. Of course, in a real world program, that would be unacceptable — we don’t want our program to “hang” forever!

However, we’re just using this approach in our first example to keep things simple. Before we’re finished this month, we’ll implement one way to avoid that problem.

2. Here, we reconstruct the 16- bit value of ADCval. As you can see, it’s a simple task.

3. In the December 2012 column, we used this equation to have a PICAXE processor convert the value of ADCval to Centigrade: tempC=( 2*ADCval–500)/10. This time, however, the Pi is doing the math.