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    Maximum Unterminated Network Speed Calculator  
    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.
    Copyright © 2010 Barani  
    INPUTS: 1 second = 1 000 000 microseconds (μs)  
    3.00e+008 m/s = Speed of light ( c )  
    % of c = Signal Propagation Velocity (Electrical signal speed in cable)  
    m/s = Signal Propagation Velocity  
    m = Network Cable Length  
    μs (microseconds) = Signal Travel time based on network cable length  
    μs = Signal Round Trip Travel time based on network cable length  
      = Number of Signal round trips for signal reflection to Damp Out  
    μs = Time for signal reflection to damp out  
    samples per bit = UART Controller Sampling Rate (Clock cycles per baud)  
    μs = Baud Length (signal pulse width)  
    Baud Rate = Pulses per second  
    Bit Rate = Baud Rate * Bits/Baud  
    Bits/Baud is dependent on Transmission Technique of device  
    A popular transmission technique is Frequency Shift Keying = 1Bit per Baud  
    Bits per Baud = Bit Rate of Transmission Technique 8bits/Byte 1048576 Bytes/MB  
    Baud Rate (Bd) Bit Rate (bps) Baud Length
    Pulse Width (μs)
    TERMINATION RESISTOR Required? Byte Rate (Bps) MB/s MB/min  
    • 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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