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Cable Run Loss (RS-485 / OSDP)

TIA-485 · Max Length vs Baud · Bus Loading

When to use: Verify an OSDP RS-485 multi-drop run. TIA/EIA-485 supports roughly 4000 ft at ≤38.4 kbps on quality twisted pair; usable length falls as baud rises (≈1700 ft at 115200). A standard RS-485 driver supports 32 unit loads. Larger gauge lowers loop resistance and voltage drop for any auxiliary power carried on the run.

Bus Parameters

Baud Rate

Cable Length

Devices on Bus

nodes

Conductor Gauge

Length Utilization

38% of 4000 ft limit at 9600 baud

✓ WITHIN LIMITS

4,000 ft

Max length at 9600 baud

Results

Length CheckOK (1500 ft)

Max Length at Baud4,000 ft

Bus Loading8 / 32 unit loads

Node CheckOK

Loop Resistance24.30 Ω (22 AWG)

Notes

Terminate both ends with 120 Ω for long/fast runs

Daisy-chain (multi-drop) — avoid stubs/star topology

Use shielded twisted pair; ground shield at one end only

Repeaters/isolators extend length beyond a single segment