Switching from positve or negative voltage to zero

Question

Hello everybody. I am using labview with my senior design project and I am running into a few problems. I am using labview to output a positive or negative voltage through the serial cable and ultimately cause an actuator to extend and retract with the simple press of a button. Using the DTR pin and RTS pin, changing them to write, and wiring in an enum to them I got it to where the voltages will switch from positive to negative. Currently if the button is not pressed the output is asserted and reads +5 volts. If the button is pressed the output is unasserted and reads -5 volts.
My question is how do you change this to instead of switching from positive to negative it switches from positive to zero or negative to zero. In the end i want to have two different buttons each reading a zero voltage when nothing is pressed. Once you press the button depending on which one then the output will be +5 volts or -5 volts. To offer a better understanding the two different pins coming out of the serial port will go into two diffent relays.  These relays will also have power converters connected to them as well as the actuator itself.
My senior design project is assistanting a handicapped individual with their pursuit of finding employment so any ideas you may have will be appreciated. Thanks in advance for any help you can offer. 

Answer 1

uncc wrote:
I am new to using labview as well as communication through the serial port so bare with me when it comes to answer these questions. I have only used labview once before to communicate to the AtMega board. 
You're missing the point. This has nothing to do with LabVIEW, and everything to do with voltages and currents. The kinds of things that an electrical engineer (including someone that's supposed to be majoring in it) is supposed to know about. The issue is that you cannot say with certainty that the "high" voltage of a serial port will be +5V. As noted in the link I showed you, it could be as high as +15V. The "low" could be as low as -15V. Thus, your circuit has to account for this if you wish to use the serial port.
From looking up info on the serial communication and from what everybody has been posting, its not as simple as intializing something to zero in labview. I know of a way to make it work but it requires using four relays instead of just the 2.
This is not brain surgery. You do not need 4 relays.
It looks like it would have been easier to use parallel port.  
Guess again. The parallel port's voltage can be +5V or +3.3V, depending on the manufacturer of the parallel port interface circuitry. The parallel port is also not designed to drive regular relays. You would need to get the exact specifications of the parallel port in terms of what its current source/sink capabilities are, and compare these to the relays you intend to use. This, though, guarantees that you will be restricted to using that exact computer. Do you really want to do this? I doubt it. That's why you need to use an interface circuit between the port and the relays. A simple Google search (as Ravens Fan suggested) will yield tons of information on this.