From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matti Aarnio Subject: Re: SCC cards Date: Thu, 30 Dec 2004 16:58:48 +0200 Message-ID: <20041230145848.GC8704@mea-ext.zmailer.org> References: <33915.203.214.147.100.1104366857.squirrel@203.214.147.100> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <33915.203.214.147.100.1104366857.squirrel@203.214.147.100> Sender: linux-hams-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: ggb112@rsphysse.anu.edu.au Cc: Linux-Hams@vger.kernel.org On Thu, Dec 30, 2004 at 11:34:17AM +1100, ggb112@rsphysse.anu.edu.au wrote: > Hello all > > I have been playing around with asynchronous serial comms on > a wireless link for some time now using mkiss. What I really > need though is a TNC with digital outputs and no need for PTT > as I do full duplex. > > I could instead try out some synchronous, FEC and HDLC SCC card > / USART that either plugs into PCI or even the async serial ports > for modest >~ 100 kbps data rates. Hopefully you do it at sufficiently high UHF so that it fits into frequency allocations.. > I see that there is a lot of information implying cards that may > anwer this descrption on the linux-ham archives. However I am > confused a bit in that I would like the card to have RS232 output > voltage levels (or at least TTL/digital). On the contrary I get > the impression that some of these cards must either plug into > ADC/DACs to feed HAM radios. I see your problem. The thing is: digital bitstream (that a HDLC speaking interface can produce) can be used fairly easily to produce a bpsk/gmsk modulation stream, but converting that stream back to bits is whole another kettle of fish.. You want this pipe in transmit side: ------ ------------------- -------------- HDLC | -->-- | FEC+INTERLEAVER | -->-- | modulation | ---> RF ------ ------------------- -------------- Doing successfull demodulation out of that stuff will actually tax heavy DSP even in moderate bitrates so that you will get correct frame out of it. (A 1 GHz Pentium-III can decode about 30 kbps link speed to data frame.) There do exist ready-made hardware chips that offer FEC+IL+GMSK modems (www.cmlmicro.com comes to mind) but they are more or less proprietary - and for fairly low datarates (narrowish bandwidths, too) http://www.cmlmicro.com/products/innovate/docs/Inn990.pdf Reading CMLMICRO's product pages, it does look like FEC+IL is something you can't quite so easily get as a stand-alone module, but CMX589A GMSK modem does exist, there you get: ------ -------------- HDLC | -->-- | modulation | ---> RF ------ -------------- Adding FEC+IL in serial form needs looking for some other hardware source. Possible to design your own FPGA logic. In usual ham environment doing packet radio goes over most inefficient possible link modulation, namely wireline modem produced audio tones are sent out with FM (of all things!) modulated RF carrier. On reception you have to have sufficiently high S/N even to demodulate the FM, let alone the payload signal. Running the same modem (with same audio interface) over SSB will allow the link-span to be 3-5 times longer with ease. Of course the SSB radios are more expensive/complex than FM ones, and you won't be getting them out of junk heaps quite to easily... However careful choice of modulations will allow recycling of FM-only intended C-class RF power amplifiers for such data radios; namely the choice will then be GMSK. These days at these modest bitrates I do suggest using USB interfaced hardware for the data. AMD has Am186CC embedded processor with USB target interface hardware, and HDLC hardware intended for ISDN BRI applications. > Any suggestions of make, model and suppliers would be greatly > appreciated. Your own lab ? A box that interfaces with full-speed USB 1.1 to your PC, presents to the PC a standard communications device profile (so that e.g. Linux can handle it as standard USB-ethernet, or whatever) and to radio side it presents 455 kHz or 10.7 MHz IF input and output -- plus PTT out, RSS in, etc. minor things. > Gerard Borg /Matti Aarnio - OH2MQK