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[pcharters]

The CM17A is meant to plug into a PC serial port. I want to use mine with an Arduino. I haven't got around to trying it yet - just trying to understand the specs. Also I use Macs and haven't had a serial port in years.
Does a PC serial port follow RS232 specs and operate off +/- 12V? Will the dongle run off +5 and 0V? I have the list of codes and the transmission protocol for the dongle. I just need to know what voltages to supply.

[pcharters]

I should add that the information I have on the CM17A makes it sound like the voltage runs between 0 and a positive value.

"While transmitting data to the FireCracker, it is important to ensure that at least one of the signals (RTS or DTR) is high at all times to supply power to the device. Should both of the signals drop to 0 at any time, the FireCracker will be reset and will require the entire signal to be transmitted again."

[Brian H]

I believe you have the protocol paper already but just in case.
http://software.x10.com/pub/manuals/cm17a_protocol.txt

Notice it does NOT use the normal transmit and receive pins. It uses the RTS and DTR signals for powering and toggling the 310Mhz transmitter.
I have seen the schematic of the CM17A {it is available on the FCC web site}. I would say a minimum of a 6 volts +- swing as it powers a +5 volt power supply through  diodes from RTS and DTR  that prevents the - part of the RS232 signal from reaching the power supply part and the controller chip has a built in set of diodes on its I/O pins. Feed by 100K resistors.

FCC Database if you care to look.
http://transition.fcc.gov/oet/ea/fccid/
Grantee B4S
Product Code CM17A
Details tab.

[dhouston]

Does a PC serial port follow RS232 specs and operate off +/- 12V? Will the dongle run off +5 and 0V?

Some PC serial ports use RS232 voltage levels and some use 0 & 5V (at least, some work with 0-5V devices). The Firecracker will work with 0-5V. I've driven it from an embedded device. It uses a PIC which has internal clamping diodes on the inputs so all that's needed to deal with higher voltages are current limiting resistors in series with the input. As you noted, you need to maintain power on one handshaking line at all times. Timing is not critical. You could even dispense with the PIC and drive the transmitter directly - it depends on what you are doing and what you're using to supply the signal. I've driven similar transmitters with just the data envelope supplying power, turning the transmitter on/off with each pulse (to save the battery) but I haven't tried it with the Firecracker transmitter section. Turn on time is all that matters and it should work with this transmitter.

• http://davehouston.org/adc.htm

If you would provide more details on what you want to do we might offer more suggestions.

[pcharters]

Thank you, Brian H and dhouston.

Using the information you provided, I now have the schematic and other information. I can see that it will run off signal lines of 5V and 0 but can also operate off somewhat higher but current limited voltages and the negative voltage is irrelevant. I will try the simple (5V) model first.

As for what I am doing with it, I want to use an Arduino to operate my X10 switched lights. Close to where the dongle will be, I have a TM751 transceiver so I should be able to do this relatively easily.

Thanks again for your prompt and useful information.