RS-485 is a popular electrical standard for data interchange over a multipoint differential bus, meaning that all of the nodes on the bus share one common transmission medium. RS-485 does not specify the maximum number of bus connections. In fact, as the total number of nodes increases, the loading on each driver will increase as well.
The Telecommunications Industry Association (TIA)/EIA-485 standard created a hypothetical unit load (UL) to help calculate the maximum number of nodes on a RS-485 bus. The standard states that a driver must be able to drive at least a 1.5-V differential signal across a maximum of 32 unit loads in parallel with two 120-Ω termination resistances at opposite ends of the bus.
You can use the two equations below to calculate the maximum number of node on your RS-485 bus. First, per Equation 1, you can express the worst-case ratio of the input voltage divided by leakage current to calculate the input resistance. After you’ve established the input resistance of the node, you can calculate the maximum number of nodes on an RS-485 bus with Equation 2:
- Input Resistance = Max (VIN / Leakage Current) (1)
- No. of Nodes = 32 / Input Resistance (2)