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 SUPPORT
FAQs
Baudrate-Calculation
USB
Calculation of Baudrates on VScom USB-COM devices
VScom USB-COM, USB-2COM, USB-4COM, USB-8COM, USB-COM-I, USB-2COM-I, USB-4COM-I,
USB-COM-I-SI, USB-2COM-I-SI
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Problem:
One question asked again and again is about supported baudrates. In most
cases the customer wants to know, if a special baudrate is supported, which
is required by certain devices. Since there are more than 60000 baudrates
supported, there is no use in enumerating them in a list. Instead here are
instructions to calculate the required parameters for realisation. If such a
configuration can be found, the requested baudrate is supported. Otherwise
the baudrate can not be realised.
To make things easy, there is a Program
for DOS. This also operates in DOS-boxes of all Windows Versions as
well as in OS/2. Even not tested, it should also operate correct in the
DOS-Emulations of various operating systems. Just enter the requested
baudrate, and the result is the best matching configuration. If the achieved
baudrate deviation is less than 2%, a reliable data transmission is assured.
Calculation:
Maximum speed:
The maximum speed with USB-COM from VScom 3000000 Baud, i.e. 3
Megabits/s. This is a theoretical value, it is not usable in most real world
situations, The line drivers for RS232 can not transmit these bitrates in a
reliable way, especially not on cables with realistic length. When using the
RS422/485 devices, it may be possible in certain circumstances. This has to
be verified in each single case.
Configuration by Division:
In all serial ports the available maximum Baudrate is reduced by a Divider. I.e.
the value is divided by a number ranging from 1 to 16384, in steps of 0.125
(one eighth). The result is the achieved Baudrate in reality. So this explains
the more than 60000 possible Baudrates of the top paragraph. In the
following calculations the maximum Baudrate is named B
(for Base), the target speed is T, the divisor is D
and the realised speed is named as S.
To check if a certain Baudrate is technically possible, one must find
the required configuration for that speed. In the first step B
is divided by T, yielding the Divisor D.
In many situations D is not an value, which can be used exactly. It is required to round it up
and down, yielding divisors D1 and D2.
The next step in calculation is to divide the maximum speed B by D
(or D1 and D2). This results in the
achievable speed S (S1 and S2).
These results have to be compared with T. If the deviation
of one of the results is less than 2%, T is a possible
configuration.
| D = B / T |
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(calculate the divisor) |
| S = B / D |
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(calculate the speed) |
| E = 100 * (S - T) / T |
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(deviation in percent) |
Of course the driver performs all of these calculations. And also the
check for the optimum value for D. The Program SPEED-USB
performs the same calculations, including the display of the deviation
(absolute and relative).
Fictitious calculation example:
Maximum speed is 3000000 Baud. The target speed is 128000 Baud.
| D = B / T |
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D = 3000000 / 128000 = 23,4375 |
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Rounding required. D1 = 23.375, D2 = 23.500 |
| S1 = B / D1 |
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S1 = 3000000 / 23.375 = 128342,25 |
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| S2 = B / D2 |
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S2 = 3000000 / 23.500 = 127659,57 |
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| E1 = 100 * (S1 - T) / T |
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E1 = 100 * (128342,25 - 128000) / 128000 = 0.27% |
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Result acceptable |
| E2 = 100 * (S2 - T) / T |
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E2 = 100 * (127659,57 - 128000) / 128000 = -0.27% |
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Result acceptable |
128000 Baud can be realised. The deviation in speed of 0.27%
does not prevent a successful transmission.
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