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It's a Rosewill RC-300 1 Port serial PCI adapter. From, it says "The Rosewill RC-300 features a 9-pin RS232 serial port to add..." so I assume it only supports RS-232? Sorry for the ignorant question; I tried looking up differences between the specifications but it's pretty complicated for me.

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You can obtain a converter. You'll get much more useful answers if you tell us what, specifically, you're trying to do. "Supports" is a very vague term. – David Schwartz Oct 30 '12 at 0:06
up vote 1 down vote accepted

Unless you've bought a card that specifically supports RS485, no, you don't have it.

RS232 uses at a minimumum, three wires. A transmit, a receive and signal ground. The signal is a swing of around +-15VDC on the receive and transmit wires and is referenced to the ground wire.

RS485 is a differential balanced signal (one wire swings positive while the other swings negative) over twisted pair, similar to what Ethernet uses and typically is used for CSU/DSU units, process control, etc. Full Duplex requires two twisted pairs.

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There's nothing in the manual to suggest the RC-300 supports RS-485

Other vendor's product descriptions generally do say if they support RS-485.

RS-485 is a more specialised comms standard than RS-232, mainly used in industrial systems

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and I don't think Rosewill makes industrial serial pci cards. – Moab Oct 30 '12 at 2:38

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