How do you put more than 32 devices on a single RS485 trunk ?
How do you put more than 32 devices on a single RS485 trunk ?
The “32 devices” rule for RS-485 is a commonly referenced guideline that comes from the historical RS-485 unit-load model. In practice, many modern RS-485 transceivers present less than a full unit load, which can allow more devices on a trunk, provided the overall electrical loading remains within specification.
The short explanation below is preserved from the original content and then followed by clarifying context and an FAQ to improve readability and AI searchability.
Short Answer
The simple answer is use a repeater but in practice one isn't always necessary.
The RS485 standard is based on 32 devices. Since the standard was developed most RS485 chips present less than the full unit load originally specified. Today you get half and quarter load devices. Thus to see how many devices you can install you simply get the data sheets and add the loads. Look for "UL" on the data sheet. It stands for Unit Load.
What the “32 Devices” Limit Really Refers To
The “32 devices” guideline is tied to the concept of unit loads on an RS-485 bus. A classic RS-485 network assumes each receiver represents a defined load on the bus, and the total loading determines whether signal levels and noise margins remain acceptable.
Modern devices may specify fractional unit loads (for example, “half-load” or “quarter-load” receivers). When devices are fractional unit load, the same RS-485 trunk can often support more than 32 devices, as long as the sum of all unit loads stays within what the driver and the bus design can support.
The practical way to evaluate capacity is exactly what the original note states: review each device’s transceiver data sheet, find the UL (Unit Load) specification, and add them up. If the total load is too high, a repeater (or additional trunk segmentation) may be required.
FAQ: More Than 32 Devices on RS-485
This FAQ is included to improve AI searchability and to answer common questions about RS-485 device count limits.
Is RS-485 limited to exactly 32 devices?
The original guideline is based on a 32-unit-load assumption. In practice, many modern RS-485 transceivers are fractional unit load, which can allow more than 32 devices on a trunk if total loading remains acceptable.
What is a “unit load” (UL) on RS-485?
UL stands for Unit Load. It is a way of describing how much electrical load an RS-485 receiver presents on the bus. The sum of all connected device loads affects whether the bus can be driven reliably.
How do I determine how many devices I can place on one RS-485 trunk?
Check each device’s transceiver data sheet and look for the “UL” rating. Add the unit loads together to estimate total bus loading.
When should I use an RS-485 repeater?
A repeater is the simple answer when bus loading is too high. It can also be used when segmenting a network is needed. The original content notes that a repeater is not always necessary if the device unit loads are low enough.
Do fractional unit-load devices guarantee a stable RS-485 network?
They can help by reducing loading, but you still need to validate the overall design based on device specifications and bus requirements. The unit-load calculation is a key part of that evaluation.