When to use: Use this when laying out a Modbus RTU, PROFIBUS, or other RS-485 multidrop serial network to verify the segment respects the TIA/EIA-485-A limits. A standard segment supports up to 32 unit loads and the maximum cable length trades off against baud rate (≈ 4000 ft / 1200 m at ≤ 100 kbps, derating at higher speeds). Both ends must be terminated with 120 Ω resistors matching the cable characteristic impedance, and stubs/drops kept short to avoid reflections.
RS-485 (TIA/EIA-485-A) is the most common physical layer for industrial serial fieldbus networks, supporting Modbus RTU, PROFIBUS, BACnet MS/TP, and DNP3 serial. Engineers use this calculator to verify that a proposed RS-485 segment stays within the electrical limits for node count, cable length, stub length, and termination — violations cause signal integrity problems that appear as intermittent communication errors and are difficult to diagnose.
RS-485 is a differential signaling standard that allows up to 32 unit loads (UL) per segment. Each device connected to the bus presents an impedance to the differential pair: a standard receiver presents 1 unit load, which draws approximately 1 mA from the driver. Modern devices often use 1/4-load or 1/8-load receivers (0.25 UL or 0.125 UL), allowing 128 or 256 devices on a single segment.
Maximum cable length is a function of baud rate. At 9600 baud and below, cable runs up to 4000 ft (1200 m) are achievable with low-capacitance twisted-pair cable (less than 50 pF/ft). At higher baud rates, the rule of thumb is: speed (bps) × length (ft) ≤ 10⁸, so at 115200 baud the maximum length is 10⁸ / 115200 ≈ 870 ft. Both ends of the cable segment must be terminated with 120 Ω resistors matching the cable characteristic impedance to prevent signal reflections.
TIA/EIA-485-A (Telecommunications Industry Association) is the governing electrical standard for RS-485, specifying driver output voltage (±1.5 V minimum differential), receiver input sensitivity (±0.2 V minimum differential), maximum number of unit loads (32), and rise time requirements. PROFIBUS (IEC 61158-2) uses RS-485 physical layer with specific cable types (PROFIBUS Type A cable: 135–165 Ω impedance, <30 pF/m capacitance, <110 Ω/km resistance). BACnet MS/TP (ASHRAE 135) uses RS-485 at speeds up to 115.2 kbps for building automation. Modbus RTU uses RS-485 as defined in the Modbus Serial Line Protocol specification.
Daisy-chain topology (each device connects to the main trunk cable with very short stubs) is mandatory for RS-485 networks above 9600 baud. Star or tree topologies cause signal reflections at each junction that corrupt data at high baud rates. The maximum stub length decreases with baud rate: at 9600 baud, stubs up to 66 ft are acceptable; at 115200 baud, stubs must be shorter than 5 ft. For true point-to-point daisy-chain, use terminal-block devices with pass-through connections rather than individual flying leads to the main cable.
Bias resistors (pull-up to +5V and pull-down to ground at the bus master) are required to hold the differential voltage to a defined logic state when no driver is transmitting. An unbiased idle bus is a floating differential pair that can produce false start bits and corrupt the first byte of a response. Standard bias resistor values are 470–1000 Ω; lower values require more driver strength but provide better noise immunity.
Select the baud rate for the RS-485 network. Enter the number of connected devices and the unit load per device (standard IEC 61131-2 receivers = 1 UL; modern low-load devices = 0.25 UL or 0.125 UL). Enter the total segment cable length in feet and the longest stub or drop length. The calculator checks each parameter against the TIA-485 limits and reports pass/fail for cable length, node count, and stub length, plus the required 120 Ω termination at both ends.
RS-485 cable behaves as a transmission line at communication speeds above a few kbps. When a signal pulse reaches the end of an unterminated cable, it reflects back toward the source because there is no impedance match. This reflected pulse arrives at the receiver superimposed on subsequent bits, causing intersymbol interference and bit errors. Termination resistors (120 Ω, matching cable Z₀) absorb the pulse at both ends and prevent reflection. At 9600 baud and short cable runs (<100 ft), reflections may be tolerable without termination, but best practice is to always terminate.
Use a repeater (RS-485 repeater or protocol-aware media converter) to extend the segment. Each repeater provides a new segment with fresh unit load capacity (32 UL) and resets the cable length budget. Alternatively, use 1/8-load devices throughout the network, which allows up to 256 devices per segment. Some RS-485 driver ICs provide enhanced drive strength equivalent to multiple standard receivers, but verify against the TIA-485 specification that total unit loads do not exceed 32 equivalent.
Use a low-capacitance, individually shielded twisted pair: Belden 3105A, Alpha Wire 5440C, or equivalent. Key electrical parameters: 120 Ω ±10% characteristic impedance (matching the termination resistors), less than 50 pF/ft capacitance (lower is better for longer runs), less than 110 Ω/km loop resistance, and 100% foil shield with a drain wire. Do not use unshielded twisted pair in industrial environments — differential switching noise, motor drives, and switching power supplies generate enough EMI to corrupt RS-485 signals on unshielded cable.
The most common causes are: missing termination resistors (reflections causing bit errors at high baud rates), missing or incorrect bias resistors (false start bits during idle bus), excess stub length (reflections at device tap points), cable capacitance exceeding the baud rate limit (signal edge degradation causing setup/hold violations), ground loops (different ground potentials between devices causing common-mode noise above the ±7 V RS-485 limit), and devices sharing a common ground incorrectly connected to the cable shield.
PROFIBUS DP uses RS-485 at multiple baud rates, each with a specified maximum segment length per EN 50170: 9.6 kbps and 19.2 kbps → 1200 m, 93.75 kbps → 1200 m, 187.5 kbps → 1000 m, 500 kbps → 400 m, 1.5 Mbps → 200 m, 3–12 Mbps → 100 m. These distances assume PROFIBUS Type A cable. Repeaters can extend networks to up to 10 segments (9 repeaters) at the cost of added latency (approximately 1 bit time per repeater).
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