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Maximum Unterminated Network Speed Calculator
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Calculate the maximum allowable network speed of a RS485, RS422
or RS232 network based on network cable length.
Determine signal travel time, and whether a terminating resistor is required
for stable network operation.
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Copyright © 2010 BARANI DESIGN Technologies s.r.o.
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INPUTS:
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1 second = 1 000 000 microseconds (μs)
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3.00e+008
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m/s
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= Speed of light ( c )
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%
of c |
= Signal Propagation
Velocity (Electrical signal speed in cable)
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m/s
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= Signal Propagation Velocity
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m
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= Network Cable Length
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μs (microseconds)
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= Signal Travel
time based on network cable length
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μs
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=
Signal Round Trip Travel time based on network cable length
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=
Number of Signal round trips for signal reflection to Damp Out
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μs
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= Time for signal
reflection to damp out
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samples per bit
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= UART
Controller Sampling Rate (Clock cycles per baud)
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μs
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= Baud Length (signal pulse
width)
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Baud Rate = Pulses
per second
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Bit Rate = Baud Rate
* Bits/Baud
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Bits/Baud is
dependent on Transmission Technique of device
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A popular
transmission technique is Frequency Shift Keying = 1Bit per Baud
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Bits per Baud
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= Bit Rate of Transmission
Technique
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8bits/Byte
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1048576 Bytes/MB
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Baud Rate (Bd)
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Bit Rate (bps)
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Baud
Length
Pulse Width (μs)
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TERMINATION
RESISTOR Required?
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Byte Rate (Bps)
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MB/s
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MB/min
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2,400
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4,800
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9,600
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14,400
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19,200
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28,800
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33,600
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56,000
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128,000
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300,000
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1,544,000
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6,000,000
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44,700,000
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155,000,000
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NOTES:
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- Default value example: 100m of cable, will require ~3μs for the reflections to dampen out in 3 round trips. This DOES NOT necessarily mean any baud width over 3μs will work. It depends on the UART controller's sampling clock rate and if the UART conroller samples 16 times per baud, we will require a 3μs * 16 =48.5μs of time per baud.
- For robustness, many people also like the noise to be at maximum 1/10th of the baud width. So, 3μs * 10 = 30μs, and since above we calculated 48.5μs, we will more likely choose the more conservative value.
- UART Network Controller Details, RS-232, RS-422, RS-485 -Wikipedia
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