THVD1552: Relationship between the increase of nodes and data rate

Hello,

The number of nodes a transceiver is specified to support is typically based on its input leakage current. This is because as leakage current increases, each driver circuit needs to increase its own output current, and this generally reduces the output differential voltage amplitude (due to the non-zero output resistance of the driver circuit). Because of this, RS-485 drivers are typically defined to meet a minimum output level when loaded by two parallel 120-Ohm termination resistors as well as 32 "unit loads" (representing the other transceivers on the network). You can read more about the unit loading concept in this blog:

e2e.ti.com/.../rs-485-basics-how-to-calculate-unit-loads-and-the-maximum-number-of-nodes-on-your-network

If the number of nodes increases beyond the specified level, then the driver may not be able to meet its specified output voltage level. Since the minimum output differential amplitude of an RS-485 driver is 1.5 V and the maximum input sensitivity of an RS-485 receiver is 200 mV, though, there is some room in the standard for reductions in amplitude. So, if an application uses a larger number of nodes than a transceiver specifies it may still work (communicate), but it may not fully comply with the RS-485 standard's guidance and may be more limited in things like cable distances supported.

Note that these are DC concerns, and they do not take into account signalling rate. In practice, though, the number of nodes will be limited by signal integrity concerns as well, and these will become more severe the higher the signaling rate. These limitations cannot be specified from a transceiver perspective, though, since they would depend on the cabling and network architecture. Common factors limiting number of nodes at higher data rates are reflections from unterminated "stub" lines (which will reflect back and interfere with the intended signal) and accumulated load capacitance from the cabling and nodes (which will slow down the transition rates of the signals and limit the maximum achievable data rate).

I hope this is all clear, but please let me know if you have any further questions.

Regards,
Max