Friday April 21, 2006

Serial ports on Sun Ultra 20

Like so many PCs today, the Sun Ultra 20 workstation doesn't come with "legacy" ports. No serial or parallel anymore, it's all USB. And for some that's a problem, like when you need to connect serial matrix printers for receipts, invoices, etc.

The Ultra 20 is based on a Tyan S2865 motherboard. But with some modifications: the COM1 port has been removed completely (although the solderpins are still there) and COM2 is only a 10 pin header.

Trying to use the COM2 port failed at first, because by default all legacy ports are disabled in the BIOS. To fix this press F2 while booting, go to the "Integrated Peripherals", select "Onboard Serial Port 2" menu and enable the port. You can leave the IO and IRQ as default (2F8 / IRQ3). Finally leave the BIOS by choosing "Save & Exit Setup".

Now you need a cable and a bracket with a DB9 connector. You can do this two ways. You can probably avoid a lot of trouble by buying this from Cables To Go. I didn't test it myself, but these seem to be the right type. If you want to go the DIY route, you must be careful. On many serial or serial/parallel I/O boards, the DB9 plugs on the bracket are connected to the PCB header, according to the right side of the diagram.

click to enlarge

The wiring on the left shows how it must be. If you would use a cramp-on DB-9 plug you're automagically good-to-go. Hoever all the cables on serial/parallel boards follow a strict 1-1, 2-2, 3-3 to 9-9 wiring, while it needs to be 1-1, 2-6, 3-2, 4-7, etc. The reason is clear when you look how a header plug is numbered differently from a DB-9 plug.

OK, long story, but knowing the solution, it's all pretty simple. Use a cramp-on DB-9 (male !!), 30 cm of flatcable, find in your junk box a bracket with a hole for the DB-9 or buy one on eBay and off-you-go.