SLUG Mailing List Archives
Re: [SLUG] Slightly OT
- To: Alex Samad <alex@xxxxxxxxxxxx>
- Subject: Re: [SLUG] Slightly OT
- From: Glen Turner <gdt@xxxxxxxxx>
- Date: Mon, 23 Jul 2007 21:24:32 +0930
- Cc: Slug <slug@xxxxxxxxxxx>
- Organization: <http://www.aarnet.edu.au/~gdt/>
On Mon, 2007-07-23 at 18:51 +1000, Alex Samad wrote:
> I found a USB to DB9M RS-232 Converter at jaycar. My problem is, the serial
> ports on the DL are 3.3v. Will these devices handle that properly ?
I don't know about the device, but generally 3.3V refers to a Low
Voltage TTL signal:
Asserted output 2.4V
Asserted input 2.0V
Non-asserted output 0.4V
Non-asserted input 0.8V
(from memory, you'd better check these)
RS-232 signals are different:
Asserted output -15V to -5V
Asserted input -25V to -3V
Undefined -3V to +3V
Non-asserted output +5V to +15V
Non-asserted input +3V to +25V
(again from memory)
Pumping a RS-232 signal into a LVTTL chip isn't going to be pretty.
And just using the output is going to be problematic too, the load
will cause current to rise beyond the spec of the output device.
So you need a LVTTL/RS-232 buffer. There's plenty of chips, designs
kits and even pre-built PCBs around. I'd have a quick hunt through
the online shops selling PICs, since 3.3V-level RS-232-protocol is
a common way of programming those.
Plan B would be the buy the part you actually wanted, a USB to
LVTTL-level RS232-protocol converter. Again, see the PIC suppliers
like Dontronics. Note that these often don't work with Linux.
I had to make one myself by buying a Linux-supported USB-RS232
dongle and ripping off its LVTTL/RS-232 buffer -- given where
you are at I wouldn't recommend that approach.
Plan C is to order a MAX3232 and make that router talk real
RS-232. Our own Rod has done that  for the WRT-54G.