How to access my 9-data bit serial port on a PC?

Question

I have developed an embedded solution which communicates over a Multi Drop Bus and now I would like to develop a PC based application which monitors traffic on the bus.

MDB supports true 9 data bits (plus start/stop/parity - and *no fudging* by using the parity bit as a 9th data bit) whereas standard Windows and Linux libraries offer a maximum of 8 data bits.

I have a StarTech PCI2S950 PC serial port card which supports 9-data bits, but am not sure how to code my monitoring app & have googled a lot to no great avail.

I would prefer to code in C (or Delphi, or C++). I have a slight preference for Cygwn, but am willing to use straightforward Windows or Linux.

Just anything to read/write 9 data bits over that PC serial port card.

Can anyone help?

Answer 1

The document at http://www.semiconductorstore.com/pdf/newsite/oxford/ox16c950b.pdf describes the differences between various UARTs. While your StarTech board includes the 16C950, which is RS-485 (and 9-bit) capable, it uses it in RS-232 compatible (550) mode, similar to 16550/8250 from IBM-PC days, and supports max 8 bit data.

You need a board with the same chip (16C950) but that exposes the RS-485 compatible 950 mode that supports 9 bit data as per the spec. And any board claiming such support would have to come with custom drivers for Windows, since Microsoft's is 8 bit only.

There are several other chips that can do 9-bit RS-485 mentioned here but again finding Windows driver support will be tricky. And, of course, many boards use 16C950 but only in 8-bit and/or RS-232 only mode, and without appropriate drivers.

In answer to your related question on Superuser, sawdust suggested the Sealevel 7205e, which looks like a good choice, with Windows driver support. It is pricey but they specifically mention 9-bit, RS-485 support, and Windows drivers. It may well be your best option.