RS-485 Serial Port Communication

The following pictures show some SuperCom specific timings ("direction" switch latency) with the RS-485 controlled through the onboard COM1 and through serial ports on a multiport PCI card.

The data monitor used was capturing data on a Notebook and was not involved in data communications.

The captured data packet was 82 bytes long ("A012345....B").

The last event shows only the RTS signal dropping.


OnBoard COM1

it took 0.36ms* = 360 µs (micro seconds)


Serial ports on a PCI card

it took 0.45ms* = 450 µs (micro seconds)

*Direction switch latency was on the above tests constantly less than 1 ms.

The switch latency in SuperCom is also related to the transmission speed (bps). We measured a latency of about 3.50ms when transmitting with 2400 bps, about 1.20ms with 9600bps and below 1 ms when higher than 9600bps. The type of serial port, the number of open serial ports and running on a busy server may have affect on the result.

The serial RS-485 multidrop data communication is mainly used to perform Master to Clients data communication - a network of up to 32 points connected through serial ports. The RS-485 multidop support, in SuperCom, is easily activated by setting the ComType or by a function call.

The RS-485 data communication is some times combined with 9-bit framing. That is something also supported by the SuperCom serial library, even a standard PC UART does not support 9-bit. 9-bit data frames do function with RS-232 also.

The above speed tests were performed using Desktop PCs with AMD Athlon 64 X2 2.5GHz and Windows XP. Newer AMD or Intel CPUs are expected to perform 2-10 times faster.

The DEMO software found online, even when activating RS-485, are working with limited functionality, especially when it comes to response timing. The complete functionality, incl. the high speed functions, is contained only in the licensed version.

Modified at: