The RS-485 bus is widely used in fields such as communications and industrial automation. In practical applications, questions often arise regarding whether pull-down resistors are needed and what resistance value is appropriate. Below, we will provide a detailed analysis of these issues.
Why are pull-down resistors needed?
According to the RS-485 standard, when the differential voltage on the RS-485 bus is greater than +200 mV, the RS-485 transceiver outputs a high level; when the differential voltage on the RS-485 bus is less than -200 mV, the RS-485 transceiver outputs a low level; When the voltage on the RS-485 bus is between -200 mV and +200 mV, the RS-485 transceiver may output either a high or a low level, but it generally remains in one state. If the RS-485 transceiver outputs a low level, this is interpreted as a start bit in UART communication, causing the communication to malfunction.
When the 485 bus is open-circuited (the 485 transceiver is disconnected from the bus) or in an idle state (all 485 transceivers are in receive mode, and no transceiver is driving the bus), the differential voltage on the 485 bus is essentially zero, and the bus is in an undefined state. Additionally, to increase the number of nodes on the bus, modern 485 chips are designed with relatively high input impedance-for example, 1/4 or 1/8 of the nominal impedance (where the nominal impedance is 12 kΩ, and 1/4 of the nominal impedance is 48 kΩ)-making them susceptible to electromagnetic interference when pins are left floating.
Therefore, to prevent the above situations from occurring on the RS-485 bus, pull-up and pull-down resistors are typically added to the bus (usually a pull-up resistor on pin A and a pull-down resistor on pin B). If an isolated RS-485 transceiver module (such as the RSM485PCHT) is used, external pull-up and pull-down resistors are generally not required because the module contains internal pull-up and pull-down resistors (for the RSM485PCHT, the internal pull-up and pull-down resistors are 24 kΩ).
1. Under what circumstances is a pull-down resistor needed?
Figure 1: Communication Circuit for Two RSM485PCHT ModulesWhen signal reflection occurs, it is typically prevented by adding matching resistors. Take one-to-one communication as an example, as shown in Figure 1. Since the 485 bus typically uses twisted-pair cable with a characteristic impedance of 120 Ω, 120 Ω terminating resistors are added at both ends of the 485 bus to prevent signal reflection.
Based on the specific parameters of the RSM485PCHT (as shown in Table 1), the equivalent circuit shown in Figure 2 can be derived, where RPU and RPD are the pull-up and pull-down resistors added to the 485 bus inside the module, and RIN is the module's input impedance.
Table 1: RSM485PCHT Specifications
Figure 2: Equivalent Circuit Diagram of RSM485PCHT Communication
Based on the above formula, the voltage difference between A and B can be calculated as
When both modules are in the receiving state, the following equations for nodes A and B can be derived based on Kirchhoff's current law:
At this point, the module is in an indeterminate state; the module's receiver may output a high level or a low level. In this case, pull-up and pull-down resistors must be added externally to the module to ensure that it does not remain in an indeterminate state when idle.
2. How should the pull-up and pull-down resistors be selected?
Figure 3: Equivalent Circuit Diagram for RSM485PCHT Communication
Assuming that the output supply voltages V̄O of the modules are the same, and since RGND are connected together, the pull-up resistors inside the modules can be considered to be in parallel. For the sake of clarity, the circuit in Figure 2 has been simplified, as shown in Figure 3. When adding pull-up and pull-down resistors externally to the modules, you can choose to add only one set or add them to each module. For the sake of clarity, we will add one set of pull-up and pull-down resistors to the 485 bus.
In this context: RPU is the internal pull-up resistor of the module, and RPD is the internal pull-down resistor of the module; in this example, both are 24 kΩ;
RIN is the input impedance of the module's receiver; in this example, the minimum value of 120 kΩ is used;
RT is the terminating resistor; in this example, it is set to 120 Ω;
RPU_EX is the external pull-up resistor connected to the module, and RPD_EX is the external pull-down resistor connected to the module;
Since the threshold level of the RSM485PCHT ranges from -200 mV to +200 mV, a voltage margin of 100 mV or 200 mV is typically allowed. In this example, a voltage margin of 100 mV is used. Based on the differential voltage formula derived earlier, the following calculation formula can be obtained:
Since the RSM485PCHT operates within a supply voltage range of 4.75 V to 5.25 V, assuming VO = 4.75 V (when the minimum input voltage VCC is 4.75 V), we obtain:
Given that RPU = 24 kΩ, we can determine that RPU_EX = RPD_EX = 461.9 Ω. Since the calculated resistance value is the maximum, you can choose to use either a single set of 410 Ω or 390 Ω pull-up and pull-down resistors on the 485 bus, or two sets of 910 Ω pull-up and pull-down resistors.
3. How to Verify the Values of the Pull-Up and Pull-Down Resistors?
The above calculations only consider that the 485 bus is not in an indeterminate state during the idle state; they do not take into account issues such as the drive capability of the 485 transceiver or the power consumption of the components used. The smaller the external pull-up and pull-down resistors, the higher the differential voltage can be maintained on the 485 bus during the idle state; however, this also increases the power consumption of the terminating resistor and the pull-up/pull-down resistors, and places higher demands on the 485 transceiver's drive capability. If these demands exceed the transceiver's drive capability, communication failure may occur.
Figure 4: Equivalent Schematic Diagram of a 485 Bus Connecting 32 Nodes
According to the RS-485 standard, when the receiver's input impedance is unit impedance (minimum 12 kΩ), up to 32 nodes can be connected to the bus. The maximum differential load for RS-485 is 54 Ω, at which point the minimum differential output voltage is 1.5 V.
As shown in Figure 4, we can see that when 32 nodes are connected to the 485 bus, the common-mode load on bus A or B is:
As can be seen, according to the RS-485 standard, the maximum common-mode load for either Bus A or Bus B is 375 Ω.
Figure 5: Equivalent Circuit Diagram of a 485 Bus with a Terminating Resistor
When the terminating resistor is added, it can be observed that the common-mode load on the 485 bus remains unchanged, but the differential-mode load decreases sharply. The differential-mode load is
Therefore, when the number of nodes on the 485 bus reaches its maximum and a terminating resistor is added, the differential load on the 485 bus remains greater than 54 Ω. According to the RS-485 standard, the minimum differential output voltage is 1.5 V.
Figure 6: Equivalent Schematic Diagram of the RSM485PCHT with 64 Nodes
Using the RSM485PCHT as an example, this section illustrates the addition of pull-up and pull-down resistors, as shown in Figure 6. The common-mode load on bus A or B is:
Figure 7: Schematic Diagram of the RSM485PCHT with 64 Nodes and Terminal Resistors
In actual testing of the above scenario, the minimum differential voltage output by the driver was 3.02 V, which is significantly greater than the minimum differential output voltage of 1.5 V specified by the RS-485 standard.
When a terminating resistor is added to the 485 bus, it can be seen that the common-mode load on bus A or B remains unchanged, while the differential impedance changes significantly. At this point, the differential-mode load is:
The calculated differential-mode load is slightly greater than the maximum load of 54 Ω specified in the RS-485 standard. We conducted actual testing on the RSM485PCHT, and its output differential voltage was 1.58 V, which is slightly greater than the minimum voltage specified in the standard.
When the differential-mode load is 54 Ω (with two 120 Ω terminating resistors connected to the 485 bus and the parallel value of the pull-up (pull-down) resistor and the transceiver's internal resistance being 270 Ω), the RSM485PCHT's differential output voltage is 1.52 V (measured value), which is essentially the same as the RS-485 standard. When the differential load is 41.54 Ω (with two 120 Ω terminating resistors connected to the 485 bus and the parallel value of the pull-up (pull-down) resistor and the transceiver's internal resistance being 135 Ω), the RSM485PCHT's differential output voltage is approximately 1.17 V (measured value), and communication is possible under these conditions. However, the maximum differential-mode load specified in the 485 transceiver chip manual is typically 54 Ω; that is, after adding two 120 Ω resistors to the 485 bus, the parallel value of the pull-up (or pull-down) resistor and the transceiver's input impedance should be greater than 270 Ω. Additionally, to ensure stable and reliable communication, the parallel value of the 485 bus's pull-up (or pull-down) resistors and the transceiver's input impedance should generally be greater than 375 Ω.
4. Summary
1) Shielded twisted-pair cable should be used for communication lines, and the shield should be grounded at a single point;
2) Avoid using terminating resistors whenever possible, unless signal reflection issues arise;
3) If terminating resistors are used, the pull-up and pull-down resistors can be adjusted to set the 485 bus voltage in the idle state so that it remains outside the threshold level range (-200 mV to +200 mV or -200 mV to -40 mV);
4) When adding pull-up and pull-down resistors, the parallel value of the pull-up (or pull-down) resistor and the transceiver's input impedance should be greater than 375 Ω.