On 03/17/2017 08:05 PM, James Carlson wrote: > On 03/16/17 23:18, Woody Wu wrote: >> Hi, >> >> I want to setup pppd to connect a Linux laptop to an embedded Linux >> device. The Linux box connected to a rs-232 to rs485 converter, then the >> convert connected via a two-wire rs-485 cable to the embedded Linux >> device's rs-485 port. > [...] >> On the otherhand, I can make success with similar commands when two >> devices connected with rs-232. So I guess, this failure was because the >> rs-485 is half-duplex. But I searched google, people seemed say pppd >> should work over a point-to-point rs-485 connection. So I want to get >> help from your experts. > > I see evidence of other problems in your trace as well. The PC is > receiving its own transmissions, which is very bad, and is the proximate > cause of the failure. It means that the converter device you're using > has local echo enabled. If there's some way to turn off local echo, you > may get a little further. Check the manufacturer's documentation for > that converter. This shouldn't happen: > > sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xb3a3da0c> <pcomp> <accomp>] > rcvd [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xb3a3da0c> <pcomp> <accomp>] Thank you James. I think the converter should be fine, without echo, since I remembered before the ppp setup, I tested it with mimicom on both devices and did not recognized any 'echo' behavior. But I like to double check it when I come to office next Monday. > > But even though that caused this failure, I don't think it's the root > problem. As another poster said, it's not going to work like this. > > Two-wire TIA-485 is indeed half-duplex. There are protocols designed > for use on it (MODBUS is one example), but right in the introduction to > RFC 1661 (PPP), it says that you need full-duplex by design. > > I think it would be possible to make it work on a half-duplex link, but > it wouldn't be simple or terribly efficient. The two schemes I can > imagine are: > > - Use a master-slave type of relationship. This means having one end > (perhaps the PC in this case) sending some sort of signal (I suggest > using back-to-back flags; two 0x7E in a row) to let the slave side > know it should send something if it has it, or to send an empty > packet. The master then just periodically polls for data or sends > what it has. Does this mean I hava to change the pppd source code or even the Linux kernel? > > - Use something like ALOHA or CSMA. Both of these require some means > of knowing that your transmitted message has been garbled by a > collision so that you can back off and retry. That might require > some electrical work on your part. > -- To unsubscribe from this list: send the line "unsubscribe linux-ppp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
