On 2017-09-02 09:32 AM, Rob Janssen wrote:
A couple of decades ago, the "GRAPES" WA4DSY 56kbit modem kit was available for a moderate price. They weren't too difficult to put together. Alignment did take a scope, but took only a few minutes. About five people in San Diego had them. As far as I know, only three ever made it onto the air here.
I remember that modem, but also I remember that there were quite some issues at that time. It was not easy to reliably make 56k HDLC using affordable hardware at that time. For interrupt-per-character serial devices the performance of a PC-XT was not quite enough. There were DMA-based controllers but they were difficult to obtain, hard to find reliable drivers for, and expensive.
We used the PI2 card here at UBC, using the original pi driver and then the dmascc driver once it was available. It was a bit of a fiddle to find a clear IRQ/DMA on crowded machines but dedicated router boxes (an old XT worked) were pretty stable. Losing ISA slots on newer computers meant that we were limited to older systems.
With two local mountain top repeaters on UHF (using custom duplexors) we had a handful of users over 70-80 km paths ... kept my dad online for several years before cables modems made it to the farm :)
Eventually the remaining users lost interest and I gave up after the last repeater went down. At $800-1000 for a working system it just didn't get enough people to sustain itself, and cheaper broadband meant many of the original users didn't find it worthwhile.
Still have two repeaters worth of gear cluttering up my storage, hard to let go of a project that took so much of my time !
... Niall