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serial-portrs485
Blue Dart
Blue Dart

Reputation: 1

Baudrate with RS485 communication in PC

I am a newbie to serial programming. I need to develop an application where a device send serial data at 250Kbps which needs to be captured through a PC. I know that the conventional RS232 COM ports have a baud rate restriction to 115Kbaud.

Can I use a RS485 to USB converter and achieve this communication?

And an additional questions if any of you guys are familiar with this topic. When I used C# for acquiring the serial data, with a conventional COM port (or even with a RS232 to USB converter) the system generated exception when I initialized the baud rate register with a value higher than 115Kbaud.

Please let me know what do need to solve my high baud rate application.

Upvotes: 0

Views: 8341

Answers (1)

Oliver Heggelbacher
Oliver Heggelbacher

Reputation: 772

even though this is an old post, here is an answer with all the technical details:

The baud rates available and usable for a Windows PC RS232 / RS485 application depend on:

  1. The baud rate range the RS232/RS485 chipset and its Windows device driver permits. For older/consumer devices, this is many times only include standard baud rates up to 115200 baud (see e.g. this information from NI). In the original post I assume the problem was indeed that such a "basic" RS232 device was used and its COM port driver itself didn't accept a setting setting higher than 115200 bit. Note also the difference between the baud rate the driver accepts (which can be pretty much everything), and the actual transmission rate on the RS232 side. See the FTDI chipset knowledgebase on achievable baud rates.

  2. The actual limits of the electrical design of the RS232/RS485 interface. The known suppliers (MOXA, Lantronix, Digi Edgeport, VSCOM, to name a few) have special products designed for industrial applications and high baud rates up to MBit range. Also FTDI itself offers complete cable sets for different baud rate rates.

  3. The electrical characteristics of your cable / connection. This includes cable length, cable shield, and for differential voltage transmissions like RS485 also proper termination with termination resistors (see e.g. this TI application note on RS485). For 250KBit I'd certainly recommend proper termination with 120 ohms termination resistors on both ends.

Finally - before starting an own programming project, I suggest to always check the COM port connection with an existing software tool. Any freeware like RealTerm or YAT, or our Docklight evaluation should be able to do this.

Upvotes: 1

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