The board appears erratic with voltages stuck high. We have noticed that not all parallel port provide enough power for the minidaq. In this case we recommend using an external power source. A 6 to12 volt supply can be used. We found it is convenient to use a 9 volt battery. For precise measurements and external power supply is best.
The board shows voltages on other A/D channels, even though nothing is connected.
This should not effect the readings of your current channel. Because of the internal multiplexing done by the A/D, other channels will float. We recommend grounding all unused analog input channels.
The board works but it doesn’t zero when grounded.
This is normal, you will get minor offset readings between all channels. These are very small and consistent offsets that can be nulled out in software. Typically, all A/D routines incorporate a calibrate procedure. Ground all A/D channels and record the readings, then add these small offsets to the final value read.
We found the Radio Shack shielded RS-232 serial jumper box (CAT 276-1403A) will fit fine on the MiniDaq. This jumper box is coated with a metallic paint that may help reduce noise if you plan to use the MiniDaq in a less than ideal environment.
Is it possible to build the board from scratch?
Yes, but we don’t recommend it, our several prototypes ended up being quite noisy, we were only able to meet the specification of the Maxim chip after we professionally laid out the PCB board.
How do I make differential readings?
There are 4 differential input channels. You must place a .1uf capacitor between the negative input and analog ground of each channel used for proper readings to be taken. The negative input signal should not vary while taking a A/D sample or the voltage read may be off.
|Negative input signal
|Positive input signal
The Mini Daq was working until I put a higher voltage than specified.
Try unplugging the Mini Daq from the PC parallel port and plugging it back in. This will reset the board and release any latch up conditions. Damage to the Mini Daq may occur if you exceed recommended input voltage.
Can the input to the parallel port be damaged?
You can damage the parallel port by not using proper TTL logic levels for the digital I/O. A good practice is to provide a buffer such as a 74LS244 in your circuit. Check out our web page for various interface circuits and projects.
How can I get more I/O?
You can run up to 4 Mini Daqs by providing 4 parallel ports for your PC. This will give you a total of 32 A/D channels, 16 TTL input channels and 28 TTL output channels
How can I further protect my Computer from my experiments?
You can use a separate parallel port ISA card for the interface to the Mini Daq. If the maximum input voltages have been exceeded, the separate parallel board provides an extra layer of protection between your circuit and the motherboard of your PC. You can purchase 3rd party add on ISA parallel port boards for $19 or less at your local computer supply store.
How are input and output ports connected to the PC?
The digital input and output signals are passed directly to your parallel port with no buffers or protection circuitry. You must observe TTL levels or provide buffers to protect your parallel port from excessive input voltage. To learn more about the PC parallel port, there is a Parallel Port FAQ located on our Web site.
Note: Digital output 6 is an Open Collector output. All other input and output pins are standard TTL logic. We have also found that some parallel ports do not implement the original specification of the PC parallel port. Digital output 6 may not work on your computer parallel port due to this.
Can I damage the A/D if I accidentally exceed the maximum input voltage?
The input to the A/D are protected by internal clamping diodes as well as a current limiting resistor. It provides a fair amount of protection. We have tested the Mini Daq with a 9volt analog input source without any problems for a short period of time.
How do I interface relays and other devices to the Mini Daq.
We have provided circuits on our web page in PDF format to do this. If you have further questions feel free to email us.
How can I increase the A/D sample rate?
Sample speed varies with the speed of your PC. The faster your PC, the higher the sample rate. Although the A/D chip is rated at 133 Khz sampling rate, sampling frequency is quite effected by software overhead and by Windows itself. You can run in a Dos environment to get better results. We have provided interface libraries to C and Quick Basic. In Labview, optimized loops will significantly increase your sample rate.
I’m getting A/D fluctuations every time I control an output port.
Connect unused A/D inputs to ground. You might be drawing too much current from the parallel port, effecting the power input to the Mini Daq. Try switching to external power such as a 9V battery.
Why does my Mini Daq not work with jumper JP4 set to INT power?
We have designed the Mini Daq with the option to get its supply voltage from the PC parallel port. Some parallel port adapters may not be able to supply enough current to fully power the Mini Daq. This is common with laptop computers that use low power 3.3 volt logic circuits. If you are experiencing this problem, set jumper JP4 to the EXT setting and connect an external 6 to 12volt supply to Pin 13 of DB25 connector P2. A 9volt battery works quite well for this.
Does your MiniDaq software driver work under Windows NT?
Our software drivers do not run under Windows NT due to the I/O protected mode features of NT. The 16 bit version of the driver, EAS12M.DLL, will work with Windows3.1 and Windows95. The 32bit version of the driver, EAS12M32.DLL, will only work on Windows95 pc’s.
What version of your MiniDaq software driver do I use for Labview?
If you are using Labview3.0, you will need to use EAS12M.DLL driver. Labview4.0 requires EAS12M32.DLL. Both of these drivers will not run under Windows NT so you will need to run Labview under Windows 95.