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I would like to connect a anemometer(RS-485, DB9) to the PC. The PC has a serial port with DB9 connector. I couldn't find the specification of the serial port. Is that RS-232 standard or RS-485? Most commonly used the RS-232, so I assume that is RS-232. Is there a possibility to configure the serial port from RS-232 to RS-485, or I just need a RS-232/RS-485 converter?

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An EIA-485 ("RS" refers to "Recommended Standard", and is an obsolete name) interface is rare on a PC, you'll need a separate interface board for that. The reason is that EIA-232 and EIA-485 are not hardware compatible, despite using the same connector type: EIA-232 uses single-ended signals, meaning that RxD and TxD are referenced to ground. EIA-485 uses balanced signals, meaning that both sent and received data have a positive and negative signal which are each other's negative.

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PC serial ports are always RS232 (originally designed for use with modems). You would need a converter, it will not be software configurable because there is different hardware required to drive RS485 lines. You can get PCI cards with RS485 on them eg http://www.brainboxes.com/pci-serial-cards/protocol/RS422-485

As an alternative, you could use a USB to RS485 converter in a lead such as this one.

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    \$\begingroup\$ The problem with the USB to RS485 converter that the USB is unreliable regarding the time, there can be more than 1s delay, we are using LIDARS with anemometer. We would like to compare the two measurement which is really time critical \$\endgroup\$ – Kicsi Mano Oct 4 '12 at 11:15
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    \$\begingroup\$ @Kicsi Mano Then give up on the PC. Your time critical makes this a task for real-time computing and you can't do that easily on a PC. Get a microcontroller, an RS-485 chip and do the time critical part of the processing on the micro. Then send the data to the PC using say RS-232 or an USB to TTL serial converter. There are plenty of microcontrollers which have two or more UARTs. \$\endgroup\$ – AndrejaKo Oct 4 '12 at 11:34
  • \$\begingroup\$ Instead of going the micro-controller route, you can use a PC; but you'll have to use a realtime OS. Windows doesn't qualify, nor do mainstream out of the box Linux distros. There are linux kernels available that are built with realtime guarantees. Alternately there are OSes build from the ground up for realtime work like VxWorks. en.wikipedia.org/wiki/List_of_real-time_operating_systems \$\endgroup\$ – Dan Neely Oct 4 '12 at 13:37
  • \$\begingroup\$ But as far as I know I can achieve more accuracy through RS232 than USB. We don't have that much data, we just measuring the wind direction and speed. \$\endgroup\$ – Kicsi Mano Oct 4 '12 at 14:46

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