Hey everybody! Today I came across a small device from around the 70's that has a mechanical counter controlled by just a small electromagnet. The way it works is you have a switch connect and disconnect from one of the poles connected to a six volt battery. My question is, how do I control this from a serial port? How many volts is a byte? Do I need to build a contraption to control it?
Here is a picture of what I'm talking about:

http://en.wikipedia.org/wiki/RS-232#Voltage_levels (although you should interpret this as "how many volts is a bit?")

I would advice about how to make a circuit but I think I'm way too amateurish for that.

For handling the COM port in DOS, please search around, surely you will find something (e.g. "serial mouse driver", "UART 16550 programming" (or UART 8250), etc.)

this kind of device is:

low frequency, don't imagine to have more than 2 inc per second.

certainly controled by pulse.

one pulse = one count.
a pulse is a signal, edge, level, shape...

to control this, using standard interface, i'd prefer lpt becaus eit covers the one bit application domain.
only one bit for the count signal.

edfed, he could still use serial in the way you propose (use a single LPT data line?), but rather than using the TxD pin, he could use the RTS or DTR pin and the wire could be extracted from a defunct COM mouse (since both signals are part of the Microsoft mode mouse protocol).

Still, voltage adaptation would be needed (if not "current adaptation" as well be means of a transistor + external power supply to provide the pulse), but although I think I have an idea of how to make the circuit, I think I'll wait for shoorick to say something first

he-he i have never operate with RS232, but as i know it has a control bit which can be set on or off directly (transmittion output is hadware generated depending on set mode and sent byte). RS232 levels are -12V for 0 and +12V for 1, another thing solenoid in mechanical counter consume sufficient current, which may exceed max load for RS232 output amplifier. if you can set any bit on RS232 or LPT, you just need something like mosfet to drive this counter. just check if mosfet you wish to use able to keep current (you may find this data written on counter or check with AVOmeter) and it is possible to send signal from pc port to mosfet gate (usually +20/-20 is allowed, but there are variations)

Thanks everyone! Hey shoorick, would you happen to know what kind of transistor I should get?

This counter is an inductive load, so don't forget about the protection diode in parallel.

yes, backward diode will be not bad i use a while only mosfets extracted from old monitors and hard drives. i've been using IRFD123 - i like it it keeps load current 1A, say, IRF630 keeps current up to 9A etc. you have to read parameters from counter (voltage and current) and search either for mosfets table, or just put parameters into google, like this. if your counter will use high voltage (100-220V) directly from wall outlet - you must use isolator, say, optocoupler like PC817, but then there will be some play with resistors