FTDI USB to TTL info needed....Help! / Computer Science / Forums

Forums

4hv.org :: Forums :: Computer Science

« Previous topic | Next topic »

FTDI USB to TTL info needed....Help!

Move Thread

LAN_403

Ash Small

Wed Nov 09 2011, 03:04PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've bought a kit from another website to enable me to plug my laptop into the diagnostic port on my car.

This requires an FTDI USB to TTL cable, which incorporates a TTL to USB chip.

I'm planning to modify the kit so that I just plug a standard USB cable into a USB port mounted in the dash, so I need to incorporate the FTDI USB to TTL chip into the kit mounted behind the dash.

I've been on the FTDI website, but I'm confused by USB to RS232 and USB to TTL. Is this the same thing, or are there differences?

Any other info relating to this project will also be useful. I'm planning to write it up and post it in the projects section once I've sorted out these problems, so we can all plug our cars into our laptops to diagnose ECU issues, etc.

Steve Conner

Wed Nov 09 2011, 03:39PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I assume both refer to the FTDI FT232xx USB-to-serial converter chips, which I've used extensively for various things.

RS232 has signalling levels of +/-12V, but TTL is 0 and 5V. The FTDI chip itself has a TTL output and needs a level shifter chip like the infamous MAX232 to convert to RS232 levels. You can buy a RS232 to USB cable using the FT232, and bypass the level shifter IC to get TTL output, or add an extra MAX232 to convert it back to TTL.

FTDI also make the FT245 which is a USB to general purpose parallel port, that could also be construed as "TTL".

The root problem is that the designations "RS232" and "TTL" are almost meaningless by themselves.

Bjørn

Wed Nov 09 2011, 03:40PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

RS-232 uses negative and positive voltages, TTL does not use negative voltages and the signal is inverted so you can convert from TTL to RS-232 using an inverting driver.

The voltage levels for either is not well defined.

Ash Small

Thu Nov 10 2011, 01:11PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the above information.

That clarifies things a lot.

What I require is a USB to TTL converter, with six pins.

Apparently the kit is designed for something like this:

but I want to use something like this:

I don't want to wait for Paypal to clesr and postage delays (usually around two weeks in total).

I've asked in Maplins, and they say they only do USB to RS232 converters, and have never heard of a six pin TTL connection.

I've looked on the FTDI website, but it seems I need to know what I need in advance (there are a lot of variations on there).

I don't even know if I need a 3.3 V one or a 5 V one.

The kit instructions just say this:

"A kit containing a PCB and all necessary components to build an adaptor to connect the vehicle diagnostic port to a standard FTDI USB-TTL cable (Not included)"

Are there any questions I should ask the supplier of the kit? Or what other information do I need in order to order the correct FTDI USB to TTL converter for this application?

Steve Conner

Thu Nov 10 2011, 01:29PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You just need to know the pinout of the magic 6 pin socket, by the looks of it, then you can bodge anything that contains a FT232xx chip up to it. I believe they are software compatible for all values of xx (BM, RL, R, and so on) - once you've installed the Windows driver they all just look like COM ports.

As to whether it's 3.3 or 5V, you have to ask your car. But surely the designer of the kit can tell you the part number of the FTDI cable he designed it to work with.

Buying direct from FTDI can't be so bad. Their head office is in Glasgow, they probably ship from here.

Ash Small

Fri Nov 11 2011, 11:59AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

He Mentions FT232RL, but the lead he reccomends isn't suitable for my application, unless I cut it, or remove the board.

Ideally I just want a board with the relevant chip/circuitry on it, rather that leads or connectors, but I'll use the second item I linked to above (which has a jumper for 3.3 or 5 volts and a USB 'B' port on it) and just remove the port, as I want to mount the port in the dash. (I'll probably use a mini port)

The information you supplied was extremely helpful, by the way. I now have a much better understanding of what I require. Thanks.

AndrewM

Fri Nov 11 2011, 06:36PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Heres another question - do the FTDI chips require the use of hardware flow control or can I use nothing but the RX/TX pins with a microcontroller UART?

Steve Conner

Fri Nov 11 2011, 06:52PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The simple answer is, yes, they behave the same as an ordinary old-fashioned 16550 type UART, you can do anything that you've been used to doing with a PC's RS232 ports.

The complicated answer is that Windows can sometimes lose some of your data if you're doing heavy duty streaming with no flow control. The FTDI driver includes an adjustable buffer that can help, and I think the chip has a FIFO in hardware too, as the 16550 did.

But the fundamental problem is that Windows isn't a real-time OS, and can theoretically lose bits of your data irrespective of what UART it's coming through, if the FIFO fills up, Windows can't service the interrupt in time, and the source can't be halted because the RTS/CTS lines aren't connected. This very rarely happens, but it can't be guaranteed not to, either.

The USB ones seemed a little worse than serial ports built right into the motherboard, but of the USB UARTs I've tried, the FTDI one always worked better for me than the Profile.

I had an installation streaming at 115200 baud nonstop for years with no issues. But ironically I used the RTS/CTS lines as it made life easier for the microcontroller I'd connected the thing to. It had a double buffered command parser so it could receive the next command while processing the previous one, but it used the RTS/CTS to stop the computer trying to stuff in a third one.

FTDI also supply a driver for Linux, Android and my own personal favourite, WinCE 6.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob