The RS-485 interface used in Foxtrot and TC800 modules (e.g., MR-0133, CP-2061, the CP-2970 and CP-2770 series, etc.)
The RS-485 interface uses a half-duplex RS-485 transceiver with a maximum data rate of 1 Mbit/s.
In terms of the number of devices that can be connected to a single bus, an important parameter is the receiver's input load:
- Receiver load: 1/4 Unit Load (1/4 UL),
- maximum number of devices per bus segment: 128,
- maximum data rate: 1 Mbit/s,
- receiver common-mode range: −15 V to +15 V,
- bus pin overvoltage protection: ±40 V.
The meaning of Unit Load (UL)
Unit Load (UL) is a unit used in the TIA/EIA-485 (RS-485) standard to express the electrical load that a receiver imposes on the RS-485 bus.
A standard RS-485 driver is designed to operate with a load equivalent to 32 Unit Loads. The number of physical devices therefore depends on how many ULs each device’s receiver represents.
The following generally applies:
| Load on a single receiver | Theoretical number of receivers |
|---|---|
| 1 UL | 32 |
| 1/2 UL | 64 |
| 1/4 UL | 128 |
| 1/8 UL | 256 |
For our systems, this means:
32 UL / 0.25 UL = 128 devices
This calculation applies to the bus’s DC electrical load, not, for example, the address space of the communication protocol used.
The definition of 1 UL
The definition of a Unit Load is based on the TIA/EIA-485-A standard.
It is important to note that 1 UL is not defined solely as a specific resistance value between conductors A and B.
The definition is based on the maximum input current that the receiver can draw from the bus across the entire required common-mode voltage range.
According to TIA/EIA-485-A, the electrical model of a single Unit Load can be expressed as a 15 kΩ resistor connected to a −3 V or +5 V source, depending on the direction of the input current. This model applies to input voltages in the range:
−7 V to +12 V
TI describes this TIA/EIA-485-A definition in its application note, “The RS-485 Unit Load and Maximum Number of Bus Connections”.
For 1 UL, the current limit can therefore be expressed as follows:
For positive current:
I_UL(V) = (V + 3 V) / 15 kΩ
For negative current:
I_UL(V) = (V − 5 V) / 15 kΩ
For example, at the extremes of the standard range:
- at +12 V, 1 UL corresponds to a current of +1.0 mA,
- at −7 V, 1 UL corresponds to a current of −0.8 mA.
It is therefore not a single constant value of current or resistance. What is crucial is that the V/I characteristic of the receiver’s input meets the applicable limit across the entire range from −7 V to +12 V.
Realistic limit on the number of devices
The figure of 128 devices should not mean that a functional RS-485 network with 128 devices can be created under all circumstances.
This is primarily the maximum number in terms of the static load placed on the receivers by the connected devices:
128 × 1/4 UL = 32 UL
In a real-world installation, the maximum number of devices or the achievable communication speed may also be limited by:
- bus length,
- cable type and impedance,
- bus termination,
- length of individual branch lines,
- number and placement of nodes,
- cabling topology,
- differences in ground potential between devices,
- electromagnetic interference.
Especially at higher transmission speeds, it is no longer just the Unit Load DC parameter that is crucial, but also the characteristics of the bus as a transmission line.
Therefore, we interpret the figure of 128 devices as:
The RS-485 Unit Load interface of our systems allows for the connection of up to 128 devices equipped with a standard 1/4UL transceiver to a single bus segment. The actual number of devices that can be used may be lower, depending on the length and topology of the cable, the transmission speed, termination, and other electrical parameters of the specific installation.