RS485 Rule of Thumb: N x L x kB x P < 485

RS485 rule of thumb: N x L x kB x P < 485
This is for RS485 bus architectures like those for Modbus RTU, BACnet MSTP, Johnson Controls N2 and so forth.

N x L x kB x P < 485

Where

N = the number of quarter (or less) load devices. If using older full load devices then consider each device as four devices. For example three quarter load devices plus three full load devices and two sixteenth load devices would mean N = 3 + 12 + 2.

L = the length of the line in 100m multiples. For example 1000m (1000yards) means L=10.

kB = the speed in kBaud. For example 9600 baud would be kB=10

P = The problem factor. P is a number which is usually unity when all the line characteristics are well conditioned. Double it for poor biasing. Double it for poor termination. Double it again if runs are close to motors or transformers. Double it for improper capacitance. Double it for poor twisting. Double it for ground faults like poor/weak or multiple grounding.

So a 100m line with four quarter loads running at 20kB with a couple of problems P=4 (320) is probably okay. But a 1000m line with ten quarter loads running at 10kB with P=1 (1000) is likely problematic.

[Updated 2012.09.19 by rescaling L to make the arbitrary number '485' easier to remember.]