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FINE TUNING AN EMBEDDED IDEA

Part 1: The Land of BL2000
by Fred Eady

Start ı Z-Worldıs BL2000 ı C Meı ı Lock and Load ı Acquire the Voltage Data ı Transport and Display the Voltage Data ı Just the Beginning ı Sources and PDF

ACQUIRE THE VOLTAGE DATA

The first order of business is to collect the sensor data. Dynamic C Premier includes library routines designed to manipulate the Z-World BL2000 analog inputs. In addition, there are plenty of examples included with the C package that actually work! The abundance of working example code makes the development process easier, making it possible for you to put the Z-World BL2000 to work collecting voltages much quicker.

There are at least three functional ways to acquire the analog data using the Dynamic C Premier analog input library functions. One method clocks a native formatted command sequence out to the TLC2543. For the BL2000, the returned value corresponds to the voltage on the ADC input pin and is in a range of 0ı4095 (12 bits). Thatıs nice. The value returned is what Iım looking for, but the clocking of the command is not quite what I want to do here. The remaining two methods return an integer and a float, respectively, representing the analog input voltage value.

As you can see in Listing 1, I chose the integer method. The float routine actually converts the voltage to an actual voltage and the integer code throws out the voltage in raw binary format like the clocking method. In either case, I can convert the voltages to ASCII format and ship them across the Internet to the Microsoft Win2k server box. Because I know that the result has to be put into a spreadsheet cell, I like the raw format because I can use formulas in the spreadsheet to scale the raw cell value if necessary. Also, converting the float voltage value to ASCII loses the decimal point in the translation. There are tricks I can employ to retrieve the original value but Iım not trying to put on a programming clinic here. So, simpler is better, as far as Iım concerned.

Reading the Z-World BL2000ıs analog inputs is easy. All you have to do is call the anaIn() function and specify the analog channel that is to be read. In Listing 1, Iım reading the three internal reference voltages at channels 11, 12, and 13. Iım also taking readings from buffered and scaled inputs 0 and 1 and unbuffered analog input 8. Iıve included some printf code to show you the results returned by the ADC channels. Of course, the final deployed code wonıt contain the print statements. Running the Z-World BL2000 code in Listing 1 writes the text you see in Listing 2 to the Dynamic C Premier STDIO screen.

After the voltages are gathered and converted to ASCII, the next step in the process is to establish a TCP/IP session between the Z-World BL2000 and Billıs server. Using the tcp_open function, the Z-World BL2000 establishes a connection with a TCP/IP-based Visual Basic application residing on the Win2k server. There is a router and Ethernet LAN environment at the Z-World BL2000 side, which gives you the freedom to use the Ethernet capability of the Z-World BL2000 instead of dialing an ISP using phone lines and a modem to transfer the voltage readings. Just a few lines of Dynamic C Premier TCP/IP code puts the Z-World BL2000 on its local LAN. A router that is designated as the gateway device in the Z-World BL2000 socket code provides access to the Internet. In this configuration, the Z-World BL2000 connects, takes voltage readings, and sends them back to the Win2k server application.

When the data arrives at the Win2k server, the ball is in the Microsoft serverıs court. At this point, thousands of dollars of outlay for prepackaged data collection software is circumvented. Anyone who works with Microsoft Office can put my solution to work.

A simple Visual Basic program is kicked off on the Win2k server that accepts the ASCII voltage data from the remote Z-World BL2000. The VB program receives the data via some simple TCP/IP programming and puts the raw voltage data in a flat ASCII file. Semicolons delimit the voltage readings when they leave the BL2000ıs Ethernet port. The delimiter allows the readings to be imported from the flat ASCII file directly to the cells of an Excel spreadsheet. If you were wondering why I was sending voltage data unconverted as an integer instead of a float in ASCII format, now it should all become clear. Using a simple Excel macro, you can take the raw ASCII numbers and import them into the Excel spreadsheet for manipulation there. The burden of conversion and scaling is taken from the Z-World BL2000 and passed to the Excel internals.

To give Excel the correct cell formula, you must know how the Z-World BL2000 ADC inputs are scaled. The numbers delimited by semicolons in Photo 2 are from left to right:

• Negative voltage read from a serial port transmit pin
• Positive voltage read from a power supply pin
• High positive voltage read from a battery supply
• BL2000 difference of the positive and negative reference voltages divided by 2
• BL2000 negative reference voltage
• BL2000 positive reference voltage

Photo 2ı This is more for clarity than anything else. Excel recognizes the semicolons as field delimiters when importing ASCII files into a spreadsheet.

Every analog input is scaled even if it is not buffered. Analog inputs 0ı3 are buffered and scaled using an LMC660 op-amp. The Z-World BL2000 positive voltage supply is the op-ampıs power source. A 200-kilohm resistor in the feedback path is used with a 1-megohm resistor on the inverting input pin to scale the input voltage by a factor of 0.2. The 12-bit ADC IC is referenced with 4.096 VDC. This results in a one-LSB change representing 0.001 V. Adjusting this with the op-amp scaling value, each ADC step becomes 0.005 V. This results in a measurement range of 0 to 20.48 V. Because the first four ADC inputs are designed to measure negative voltages, the non-inverting inputs of the LMC660 are biased at 1.707 VDC to place the zero point in the center of the 20.48-V range. This biasing action results in a measurement range of ı10.24 to 10.23 VDC.

Letıs compute the first value using what we know about the buffered and scaled analog inputs. The number of steps is represented by the far left number, 3819 decimal. We know our zero point is halfway, or 2048 steps. The difference from halfway is 2048 ı 3819 = ı1771. Each step is equivalent to 0.005 VDC. So, ı1771 ı 0.005 = ı8.855 VDC. Using my uncalibrated digital VOM, I measured ı8.863 V across the screw-downs connected to the serial port transmit pin.

We already know the next voltage must be positive because the bit count is less than 2048. Using the same algorithm from the previous example, you can compute the distance from relative zero as 2048 ı 443 = 1605. Applying the scaling value of 0.005 V per step, you come up with 1605 ı 0.005, or 8.025 VDC. Again, applying my uncalibrated measuring stick, I read 7.949 across analog input 1 to analog ground. Both readings contain a 1% error factor when the actual uncalibrated VOM reading is compared to the BL2000 output.

The third bit reading was taken from a pack of five 9-V batteries connected in series. Channel 8 of the Z-World BL2000 ADC subsystem is rated for a dynamic range of 0 to 49.15 VDC. This equates to 0.012 V per bit. The scaling factor is achieved using a voltage divider consisting of an 11.0-kilohm 1% input resistor in series with a 1.0-kilohm 1% resistor to analog ground. The ADC input is taken from the center point of the voltage divider. You can now compute the battery pack voltage as follows:

3865 ı 0.012 = 46.38 VDC

Applying my uncalibrated VOM reading of 46.68 V as the absolute reference again puts the Z-World BL2000 reading of 46.38 V within the 1% accuracy area.

The final three readings are the internal ADC reference voltages. A bit count of 2048 represents half of the reference voltage of 4.096 VDC using a bit value of 0.001 V. Moving one number to the right, ground is 0 V. Finally, the reference voltage is sampled and, as expected, all 12 bits of the ADC fire off to give you 0xFFF, or 4095 decimal. Again, the actual application will not contain the pretty VB window, but the contents of the VB window are written to the file just as you see them in Photo 2. Listing 3 is the tale of the tape for the VB code.

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