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Dataq Modifications
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Here are several easy modifications for the Dataq DI-194RS or DI-154RS. I have seen two different styles: the kind that come packaged and the kind that come as an unpackaged PCB. I believe the unpackaged ones are what you get when you are fortunate enough to get Dataq’s free PCB. These instructions cover the packaged 194RS and 154RS only. The one unpackaged board that I have seen had a completely different power section. The unpackaged model was marked “DI194 Rev G” on the PCB, the packaged model marked “DI194/154 Rev G”. The mods shown here are independent of each other so make as few or as many as you like. No special tools are needed for any of these mods. The modifications outlined below include:
- Adding an external power supply for reliable operation with a laptop. This can be done in two ways:
- external power only
- or automatic switching between external and serial port power.
- Adding a “Power On” LED.
- Convert one or more channels from ±10V bipolar input to 0-10V unipolar input.
- Convert one or more channels from ±10V bipolar input to 0-5V unipolar input.
Why convert channels to unipolar? Because any sensor that you are using that is based on a bridge circuit only needs a unipolar input and you’ll also double your converter resolution by going to unipolar. Loadcells, strain gages and pressure sensors are all examples of bridge based sensors that will benefit from unipolar conversion. There are instructions on calibrating the Dataq when used with a loadcell at www.solid-motion.com.
Below is a schmatic outlining the modifications. You can also download it and view it with ExpressPCB schematics. Dataq also has a decent schematic you can view that shows the entire DI-194RS circuit minus the RS232 transmit/receive level-translation/buffer circuit.
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Step 1. Remove the two screws on the back and the PCB assembly will pop right out.
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Here’s the whole assembly
Notice R24 (black surface mount resistor) right below the grey potentiometer. We’ll remove R24 in a moment. R1 (light blue surface mount resistor) is upside down at the lower right.
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Step 2. To convert a channel from ±10V bipolar input to 0-10V unipolar input, one resistor per channel has to be removed. In the picture above, R1 is removed to make channel 1 0-10V unipolar input. It is quite easy to remove a surface mount resistor with a normal low wattage soldering iron, but it takes a little practice. First place the soldering iron tip on one side of the resistor for one or two seconds. Then quickly move the soldering iron to the other side for one or two seconds. Now move the soldering iron tip back to the first side for about half a second and then give the tip a quick flick to the side. The resistor should come right off. The overall sequence is: first-side, second-side, first-side, flick.
There is one other bipolar/unipolar option. Jumper JP2 is set by default to the “Bipolar” setting for ±10V bipolar input. Moving the jumper to the position closest to the connector will give you “Unipolar” 0-20V input for all channels. Once resistors are removed though, it doesn’t matter what the jumper setting is only for the channel(s) for which you have removed the resistors.
Follow this chart for modifying whichever channels you desire.
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Step 3. Now, on to the external supply. First remove R24 as shown above. If you will only be using an external supply you can skip Step 4. Dataq documents another approach to adding external power, put it involves adding surface mount components and the mod must be made for a specific battery voltage. The approach documented here is easier to perform and is more flexible, but it does require an additional connector.
Step 4. This step adds a diode so that you will have automatic switching between an internal and external supply. Add a 1N914 diode in series with a 100 ohm resistor across the pads that R24 used to sit on. The stripe side of the diode (cathode) should be connected to the resistor. A 1/8 watt resistor is a good size to use here because it is so small, but a 1/4 watt will also work if you bend the leads carefully. Trim the leads and bend them at a right angle so that they sit on the pads as shown below. This diode and resistor will allow either the serial port power to be used or external power. When external power is applied the diode will block the external supply from trying to put power back onto the serial port.
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Step 5. Before adding the external power wiring, let’s add an (optional) LED for “Power On” indication. The LED will light when the PC’s serial port is enabled or external power is applied to the Dataq. Conveniently, Dataq has a couple of unused pads marked C1 that are perfect for adding an LED. Wire a 470 ohm resistor in series with the LED. The right hand side of the LED is the long lead and goes to the +5V pad on the right of the photo below. You can bend the whole assembly over to reduce the height. The picture below looks a little funny because you are looking down the length of the bent-over 470 ohm resistor. We’ll stabilize it with a dab of hot melt glue at the final assembly stage.
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Step 6. Now we are ready to add the wiring for the external power connector. Below is an overhead photo with the holes that will be used circled in black. You can also see the circled holes in the photo for Step 5. The hole on the right is for the ground lead and is right next to the pad used for the LED so the hole is a little hard to see. The pad on the left is for the positive lead. The holes will nicely fit a 26 gauge solid wire. A short piece of solid wire should be connected via a wrap or two to a beefier 22 gauge stranded wire that goes to the connector.
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The connection between solid and stranded wire is covered with shrink tubing.
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The LED and diode resistor are then cemented in place with hot melt glue. RTV will also work. Use a dab of glue to cement the diode and resistor added in Step 4 to the side of the gray potentiometer.
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Step 7. You can drill a hole in the cover so that you can see the LED or even raise the LED assembly high enough so that it sticks through the cover. Next, add a connector for the external power which can be any convenient connector. I used a Molex 1625-2PRT that I bought at Fry’s Electronics. The 1625-2PRT is a kit that has the following parts which are also available from Digikey:
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A resistor needs to be put in series with the positive side of the power connection as given in the table below. If you put the resistor on the battery side of the connector, you can make up several connectors for several different battery voltages for maximum flexibility. Otherwise you can put it on the Dataq side of the connection.
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Drill a hole or mill a slot in the case to allow the external power wires to pass through and you’re all done! If you modify your Dataq, send me a picture and describe what you did and I’ll post it.
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Copyright 2003 Solid-Motion www.solid-motion.com Reprinted by permission