FTDI interface and Cochran EMC-20H

[Jef Driesen]

On 2014-06-10 00:27, John Van Ostrand wrote:
> I recently purchased a Cochran EMC-20H which uses a FTDI USB chip to
> interface with the dive computer. I'm trying to initiate a conversation
> with the device using my own code to see if my understanding of the
> protocol is correct. I'm not getting what I expect and I think it has 
> to do
> with my lack of understanding how the FTDI chip normally interfaces 
> with
> devices like this. Is there anyone who can help me figure out what is 
> wrong
> with the communication method?
> 
> I've tried:
> 
> cu -l /dev/ttyUSB0 -s .... -e -o and I get a stream of the same wrong
> character (~).

I doubt this will work. Most dive computers use a request/response type 
of communication, which means you have to send/receive specific data 
packets. I could be wrong, but I don't think cu can do that.

> libftdi using ftdi_write_data and ftdi_read_data and I'll get FFs some 
> 00s

In theory using libftdi should work, although I haven't tried that 
myself. Note that there is currently someone porting libdivecomputer to 
libftdi for use on Android. So if you would like to go that route, it 
would a good idea to share knowledge.

> C language write/read using serial interface

That's what libdivecomputer is doing. I suggest you start with this. 
Have you already tried using the libdivecomputer serial communication 
code?

> I've captured a conversation using USBLyzer on Windows 7 and if my
> understanding of the data is correct I know some of the protocol but I 
> have
> yet to receive anything that looks like good data using any of the 
> methods
> I've tried.

The ftdi chips use some data bytes (two bytes per usb packet if I 
remember correctly) to communicate status. If you capture the usb 
communication, you'll capture those status bytes too, but they are not 
part of the dive computer communication. To reverse engineer the dive 
computer communication protocol, it's easier to capture the serial 
communication instead, because then you no longer have to deal with the 
USB and/or ftdi overhead.

Jef