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[admeeral]

I have a self designed system to which I am adding some x10 capability.
At the moment I am adding a USB parallel interface and plan to include a Firecracker port.
This device is intended for use with the old and rapidly becoming extinct serial port
However it only uses the two lines RTS and DTR.  Since it only receives it may not
need the full +15, -5 voltage swings of the original interface.  Can anyone tell me if
it will respond to CMOS levels +5, 0?

Muchas Gracias

[pconroy]

I don't know the answer - we'll get that out'a the way first...   

I do have a couple of USB/RS232 cables working for a handful of my legacy serial devices.
Since USB tops out at 5V, IIRC, I'd think that would push me towards a guess.

[Brian H]

ftp://ftp.x10.com/pub/manuals/cm17a_protocol.txt
My serial is kind of rusty. Not sure if - or + is what the protocol information calls for as high.

Part of this X10 project used a Firecracker Interface. Maybe it can also give you some clues to what signal levels where.
http://www.kronosrobotics.com/Projects/x10.shtml

[admeeral]

Thanks for the replies but, of course, I still do not know the answer.
When you just plug things together the levels are not published.
I have decided to interface via a couple of optocouplers and then I
can change the output levels to the firecracker by simply adding an
additional dc power input.

I suppose how much RF power output this baby has also is not known.
From the FCC specs of 200uV/M at 10 M a maximum of about 1mW
at the transmitter antenna would be about right.  Does this baby have
much of an antenna?  I suppose I could just dismantle it and see I guess.

admeeral

[Brian H]

How about schematics and photos?
FCC database.
https://fjallfoss.fcc.gov/oetcf/eas/reports/ViewExhibitReport.cfm?mode=Exhibits&RequestTimeout=500&calledFromFrame=N&application_id=93404&fcc_id='B4SCM17A'

Schematic looks like it uses the positive level of the RS232 to generate +5 volts and data for the PIC12C508 controller chip. The two 100K resistors and the chips internal protective diodes clamp the negative and positive RS232 voltages to a safe level on the inputs to the chip.

I believe the antenna is that small loop near the silk screened H10616A 3

Take a look at the RF section. It is one transistor used for both RF oscillator and output. I doubt it has much power output.

[Charles Sullivan]

I have a self designed system to which I am adding some x10 capability.
At the moment I am adding a USB parallel interface and plan to include a Firecracker port.
This device is intended for use with the old and rapidly becoming extinct serial port
However it only uses the two lines RTS and DTR.  Since it only receives it may not
need the full +15, -5 voltage swings of the original interface.  Can anyone tell me if
it will respond to CMOS levels +5, 0?

Muchas Gracias

The CM17A Firecracker will work with a USB-Serial adapter which supports the serial status lines.  These will generally swing +/- 5 Volts.  (Note that some of the el-cheapo adapters from Hong Kong do NOT support these lines, only TX and RX.) 

Provided your CMOS can supply as much current as a serial port, it ought to be able to power the CM17A lines "On" with +5 Volts.  Whether 0 Volts is low enough for the CM17A to consider the line as "Off" (or pull it down fast enough), I can't say.
 
BTW, the CM17A is a transmitter, not a receiver.

[Brian H]

0 volts should be fine. The negative RS232 level is clamped with an internal protective diode in the controller chip.

[Charles Sullivan]

0 volts should be fine. The negative RS232 level is clamped with an internal protective diode in the controller chip.

The diode would provide approximately -0.5 Volts versus the 0 Volts (or whatever the actual "off" level is for the CMOS).   I imagine some experimentation will be required to know for sure whether it will work.
 

[-Bill- (of wgjohns.com)]

Just my 2 cents.

As far as I remember, the RS232 spec specifies 12 volts for the signal lines, but the RS232C spec that is more frequently supported on PC's (especially 9-pin plugs) is 5 volts.

Or am I "missmembering"?   

 

[Charles Sullivan]

Just my 2 cents.

As far as I remember, the RS232 spec specifies 12 volts for the signal lines, but the RS232C spec that is more frequently supported on PC's (especially 9-pin plugs) is 5 volts.

Or am I "missmembering"?   

 

The original EIA RS232C doesn't specify a particular value but allows anything between +/- 3 to 25 Volts.  Between +3 and -3 Volts is an undefined zone.

The IBM PC Asynchronous Communication board specifications limited the swing to +/- 15 Volts maximum and used +/- 12 Volts from the system power supply, which became the de facto "RS232 standard".   It was just a hardware spec and there was no minimum swing defined as in the EIA spec.

[admeeral]

OK so I went and built the interface.
It uses an ELEXOL USBMOD245A interface which basically translates
a USB output to a parallel output.  I use Opto couplers to connect the two power
systems and I have the capability of inputting +15/-3 down to +5/0 simply by coupling
to  different wall transformers.  Works at 5 and has more power out at 7 V.  "Negative"
set at 0.  I also use the other 6 outputs for an environmental interface.

If the Firecracker has a loop antenna then the output will have circular polarization.. not the best
since all the receivers have linear antennas. 

The hand held transmitters have a loop antenna also.   The CM15A has a internal wire antenna and
has much better range.

Has anyone tried replacing the loop antenna on the Firecracker with an external 1/4 wave dipole??

[Charles Sullivan]

OK so I went and built the interface.
It uses an ELEXOL USBMOD245A interface which basically translates
a USB output to a parallel output.  I use Opto couplers to connect the two power
systems and I have the capability of inputting +15/-3 down to +5/0 simply by coupling
to  different wall transformers.  Works at 5 and has more power out at 7 V.  "Negative"
set at 0.  I also use the other 6 outputs for an environmental interface.

If the Firecracker has a loop antenna then the output will have circular polarization.. not the best
since all the receivers have linear antennas. 

The hand held transmitters have a loop antenna also.   The CM15A has a internal wire antenna and
has much better range.

Has anyone tried replacing the loop antenna on the Firecracker with an external 1/4 wave dipole??

You can see the innards of the CM17A on the FCC website:
  https://fjallfoss.fcc.gov/oetcf/eas/reports/ViewExhibitReport.cfm?mode=Exhibits&RequestTimeout=500&calledFromFrame=N&application_id=93404&fcc_id=%27B4SCM17A%27

[admeeral]

Thanks Charles. 
All the info anyone wanted at one web site.
The schematic was most helpful.

Admeeral

[Charles Sullivan]

Thanks Charles. 
All the info anyone wanted at one web site.
The schematic was most helpful.

Admeeral

If you succeed in adding an external antenna to the CM17A, please post your results and method here.

The best I could think of trying for improving the range was to mount the CM17A up high at the end of a serial cable and using the passive coupler trick:
http://jvde.us/x10/passive.pdf

[admeeral]

Thanks Charles.

The biggest problem I see is that since there is no crystal or adjustable components it would be
easy to shift the frequency.  My unit transmits at 310.15MHz so I suppose the receivers have sufficient
bandwidth for that offset.  When I get everything together I'll check the range.  Since I have a USB adapter
it is simple to mount it in an optimum position.  Let you know. Mounting it on the back of a computer would appear
to be problematic.

Admeeral