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Baud Rates Supported by the Windows Drivers

The USB232U and USB485Ui are updated versions of the USB232 and USB485i (although they are currently still sold by us as USB232 and USB485i). They support more baud rates than the older devices and have other features, such as more control over latency. The latest Windows drivers are required if the extra baud rates or other features are to be used.

For the USB232U and USB485Ui USB to serial converters, based on the FT232BM or FT232BL USB UART, the following baud rates are supported by the current Windows drivers with an accuracy of 3% or better (typically better for lower baud rates). The ranges shown to the left outside the parentheses are the requested baud rates and the ranges shown to the right in parentheses are the corresponding extremes of the actually generated baud rates:

Baud Requested (Baud Generated)
184 - 1369863 (184 - 1333333)
1373878 - 1449275 (1411765 - 1411765)
1456311 - 1545000 (1500000 - 1500000)
1941748 - 2060000 (2000000 - 2000000)
2912622 - 3090000 (3000000 - 3000000)

If the Windows driver is requested to use a baud rate outside the above ranges, it will generally default to using a baud rate of 115200 (actually 115385). The following specified ranges are an exception to this rule, due to hardware restrictions. The values shown to the right of the '=>' sign are the baud rates actually generated for the corresponding ranges specified on the left:

Baud Requested (Expected) Generated
1557552 - 1648351 (1600000) => 2000000
1664355 - 1765713 (1714286) => 2000000
1797914 - 1907406 (1846154) => 2000000
2126001 - 2255474 (2181818) => 2000000
2330098 - 2472000 (2400000) => 2000000
2600555 - 2758928 (2666666) => 2000000

Custom Baud Rates in Windows Applications

Please see Setting USB232/485i Custom Baud Rates in Windows for more information about using custom baud rates in Windows applications.

Baud Rate Generation

The actual baud rates that the device can generate are given by the formula:

Baud = 24000000 / N

where N is any integer in the range 8 to 131071. However, the UART does not generate the correct baud rates if N = 9, N = 10, N  = 11, N = 13, N = 14, or N =15  (the UART behaves as though N = 12 in these cases).

Conversely, the formula may be reversed to produce a baud rate divisor as follows:

N = 24000000 / Baud

However, note that the same constraints apply for valid values of N.

If it is planned to spoof an application supported baud rate to another baud rate not supported by the application, as described in Spoofing Baud Rates for USB232 and USB485i, the required divisor value (with integral and fractional parts) for the process is N / 8.

Latency and Data Throughput

FTDI's Application note AN232B-04 discusses latency and data throughput issues for the FT232BM or FT232BL USB UART used by the USB232U and USB485Ui devices. This applies to the new USB232U and USB485Ui devices, based on the FT232BM or FT232BL USB UART. There is a latency timer that defaults to 16 ms, but can be changed to any value from 1 ms to 255 ms in 1 ms increments. In Windows Device Manager, this can be set using the "Advanced Settings" dialog for the port.

Regarding the application note, please note that the USB485i and USB485Ui do not support hardware flow control.

On the USB side, the devices appear as "Full Speed" (12 Mbit/s) USB devices.