On Mon, Jul 23, 2007 at 09:24:32PM +0930, Glen Turner wrote: > 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. My fault, not talking the right lingo - but I new I was on the right list
> > 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. This is what I gathered from the openwrt web site, they also mentioned that you could buy the prebuilt ones, or use some old mobile phone data cables. > > 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 [1] for the WRT-54G. > a friend has offered to build one > [1] http://www.rwhitby.net/projects/wrt54gs/ > >
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