LaCrosse WS-23xx (and others) Serial Port Operation

315MHz and 433MHz wireless connections are very commonly used by consumer electronics including weather stations, home automation remote controls, power consumption meters, car alarms, and also many DIY projects.
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rachalmers
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LaCrosse WS-23xx (and others) Serial Port Operation

Post by rachalmers » Tue Jul 29, 2014 3:23 pm

This is an indepth discussion, with PDF (in colour) attached, by Al Testani, from the Yahoo Groups, LaCrosse Weather Stations. Forever indebted. Thanks Al. Brilliant work.

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LaCrosse WS-23xx (and other) Serial Port Operation 
Al Testani 
10/18/2011 
This write-up is a compilation of what others have contributed on the subject and my own 
experimentation and design. 
I needed to understand exactly how this worked to be able to design a pair of radio modems to allow at 
least ¾ mile separation of the console and the Internet connected computer. The modems are built and 
operating (on the bench at this point) so the information below is correct. 
The WS-23xx stations and probably other LaCrosse models use a strange physical serial communication 
scheme. The cable pinout is as follows (thanks to na1db for posting the following to the LaCrosse 
Yahoo group): 
On RJ-11 Side (clip down, telephone style connector) 
From left to right looking down on the connector 
Color: Blue Brown Green White 
On the 9 Pin D-Shell (PC Interface) 
Signal: RTS RXD TXD DTR 
Pin...: 7 2 3 4 
So Blue -> Pin 7, Brown -> Pin 2 etc. 
Note that there is NO connection 
to the RS-232 ground (pin 5)!!! 
A diagram of each end of the cable looking from the front: 
DB9 
‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 
\  5  4  3  2  1  /
\   9  8  7  6  /
   ‐‐‐‐‐‐‐‐‐‐‐‐‐‐   
RJ11 connections to DB9
          |‐‐|  (tab)
‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 
|                           |
|  7 | 2 | 3 | 4    |
‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐                     
RTS and DTR are typically used for handshaking in a serial link but are NOT used for that purpose here. 
Lacrosse took this unique approach to serial communication to save a few cents in the hardware and 
while it does work with "real" com ports on PCs and the better grade of USB to serial adapters, it is a 
cheap kludge of a way to implement serial communications. It appears that to avoid the cost of a MAX232 type device (used ubiquitously for RS-232 connections) 
and 6 capacitors, what they did was to "steal" +V and -V from the PC or USB to serial adapter if it 
supports all the handshake signals. By setting RTS to +V and DTR to -V they effectively have these 
voltages and a virtual ground between them and they got it all for free. Quite clever, actually, but it has 
caused hassles for lots of people. 
For receiving commands from the PC they compare the incoming RXD signal (which swings +/-) to the 
virtual ground. For responses transmitted back to the PC they simply connect either the RTS or DTR 
"voltages" to the TXD pin. There is NO handshaking going on in this interface. It is a simple Rx/Tx 
interface. 
In the simplest terms one could implement the “DTE” side of a link (e.g. from a microprocessor, radio 
modem, etc.) as follows: 
For direct computer connections the polling software (e.g. WUHU, HeavyWeather, etc.) sets RTS to a 
high voltage level and DTR low so communication can commence. 
A complete circuit implementation is like this: This is how I did it for the radio modems I developed and it works well. 
For those who aren’t engineers and wonder how this works, the circuit operation is as follows: 
The MAX3232 IC uses one input voltage (VCC = 3.3V) and creates +/- voltages since RS-232 requires 
signals that swing +/-3V to +/-15V. In the case above, the MAX3232 creates about +/-6V. The overall 
operation is that the IC receives RS-232 level signals and converts them to logic levels (approx. 0 to 3V) 
and vice versa for outputs. So the RXD and TXD signals are logic level signals and the TXOUT and 
RXIN are RS-232 level signals. 
The +/- 6V the IC creates are available on pins so the RTS_high signal is simply connected to the +V 
pin and puts +6V on pin 7 of the DB9 connector. For the DTR_low signal, -6V is required on DB9 pin 
4. In this case I used one of the outputs of the IC (T2out) instead of tapping the V- pin directly. All of 
the gates in the MAX3232 are inverting so this is why 3.3V is connected to T2in which is inverted and 
results in -6V on T2out. 
That’s all there is too it. 
I hope this information proves useful in: 
 
1) Understanding why some USB to serial adapters don’t work where others do. It is simply that the 
bad ones do not connect anything to RTS, DTR, etc. or any other pin besides Rx and Tx (and 
ground). 
2) Having a clear picture of how LaCrosse implements the serial port on their units. 
3) For DIY projects, have the equivalent circuit and an example application circuit to refer to. 
Thanks to all who have worked on this problem in the past and supplied information which I was able to 
use. 
Attachments
-LaCrosse Serial Port.pdf
(45.82 KiB) Downloaded 868 times

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