Serial Cable Length and Signal Integrity Calculator
Four mode workstation for serial cable length, signal integrity, timing budget, and noise margin across RS-232, RS-422, RS-485, CAN, and I²C with standards compliance against TIA-232-F, TIA-422-B, TIA-485-A, ISO 11898, and I²C UM10204.
What the Serial Cable Length and Signal Integrity Calculator does
A serial cable that works fine on the bench can fail in the field for a reason that is not obvious from the wiring alone. It is too long for the baud rate, its capacitance has rolled off the edges of the bits, the termination does not match the line and the reflections are closing the eye, the round trip delay no longer fits inside the bit period, or a ground potential difference between the two ends has swamped the differential receiver. Each of these is a different physical mechanism with a different limit, and a Modbus RTU run, an RS-232 console cable, a CAN vehicle backbone, and an I²C bus each have their own dominant constraint. Treating them all as one length rule is how a commissioned link drifts to intermittent.
Read the full overview
The noIM₃ Serial Cable Length and Signal Integrity Calculator is a four mode workstation for sizing and validating serial cables on commissioning, integration, and field service work. Length mode solves the maximum reliable cable length for the chosen standard against three constraints at once: the bit time / propagation delay rule (cable propagation delay typically under one tenth of a bit time), the cable capacitance limit (TIA-232-F 2500 pF for RS-232, I²C 400 pF), and the standard length / baud product (TIA-422-B and TIA-485-A length-baud curve, ISO 11898 CAN bit time × length product). The tightest constraint wins and is named, so the engineer knows whether the link is capacitance limited, timing limited, or standard limited.
Integrity mode computes the RC rise time at the receiver, the resulting bandwidth, the reflection coefficient at the termination, and the eye margin against the bit period. Timing mode reports the one way and round trip propagation delay, the bit time, and the timing margin. Noise mode builds the cumulative differential noise budget at the receiver — induced noise reduced by the CMRR, ground potential difference reduced by the shield attenuation, and the IR drop along the cable resistance — and returns a PASS, MARGINAL, or FAIL verdict against the configured receiver sensitivity. A datasheet anchored cable library and a reference panel of auditable formulas back every result, and every mode renders the Length vs Baud envelope with the operating point overlaid.
Capabilities 8
Multi standard length and integrity coverage
Single workstation covering RS-232 / EIA-232, RS-422 / EIA-422, RS-485 / EIA-485, CAN Bus per ISO 11898, and I²C Fast mode. The maximum length, the integrity, the timing budget, and the noise margin are computed against the constraints that apply to the chosen standard: the capacitance limit for RS-232 and I²C, the length / baud product for RS-422 and RS-485, the bit time × length product for CAN, and the RC rise time and reflection coefficient for all. The reference panel surfaces the standard limits side by side so the result can be cross checked against the published limit.
Length mode — tightest constraint wins
Length mode solves the maximum reliable cable length against the bit time / propagation delay rule (cable propagation delay typically under one tenth of a bit time), the cable capacitance limit (TIA-232-F 2500 pF for RS-232, I²C 400 pF), and the standard length / baud product (TIA-422-B / TIA-485-A length-baud product, ISO 11898 CAN bit time × length product). The tightest constraint is reported alongside the safe operating length, so the engineer knows whether the link is capacitance limited, timing limited, or standard limited.
Integrity mode — RC rise time, bandwidth, and reflections
Integrity mode computes the RC rise time at the receiver from the driver source impedance, the cable capacitance, and the length (t_r ≈ 2.2 · R · C). It reports the resulting bandwidth (BW = 0.35 / t_r) and the reflection coefficient at the termination (Γ = (Z_t − Z₀)/(Z_t + Z₀)). The eye margin is computed against the bit period using the driver rise time and the driver output voltage as inputs, so the configured driver and the configured cable are evaluated as a pair rather than against generic limits.
Timing mode — propagation delay and bit time budget
Timing mode computes the one way propagation delay from the cable length and the velocity factor, the round trip propagation delay (relevant for half duplex protocols where the master must wait for the slave reply), the bit time at the configured baud, and the maximum cable length under the bit time rule. The timing margin between the propagation delay and the bit period is reported in nanoseconds and as a percentage of the bit time, so a borderline link is identified directly.
Noise mode — cumulative differential noise budget
Noise mode computes the cumulative noise budget at the differential receiver. The induced noise from adjacent cables is reduced by the CMRR (common mode rejection ratio) of the differential receiver, the ground potential difference between the two ends is reduced by the shield attenuation, and the IR drop along the cable resistance is added. The final differential voltage is compared against the configured receiver sensitivity (typical 200 mV for RS-485, but configurable per part), with a verdict that flags the link as PASS, MARGINAL, or FAIL.
Cable library with datasheet anchored defaults
Cable library covers Cat5e / Cat6 UTP, shielded twisted pair, Belden 3105A / 9841 / 8132 RS-485 reference cables, low capacitance RS-232 cables, CAN cables, and I²C cables. Per type capacitance (pF/m), characteristic impedance (Z0), velocity factor (% c), and DC resistance (Ω/km) populate automatically from the library. Any value can be overridden for non standard cables or measured data so the result reflects the actual cable on site rather than the library default.
Length vs Baud envelope chart with operating point overlay
Every mode renders the Length vs Baud envelope for the chosen standard with the configured operating point overlaid so the safe regime is visible at a glance. The envelope shows the standard length / baud product curve and the bit time / propagation delay curve, with the tightest envelope highlighted. Operating points inside the envelope are safe; operating points outside the envelope are flagged with the constraint they violate.
Auditable formulas and copy / print outputs
The reference panel documents the formulas (t_r ≈ 2.2 · (Z₀/2) · C · L, BW = 0.35 / t_r, Γ = (Z_t − Z₀)/(Z_t + Z₀), L_max under the prop < T_bit/10 rule) and the cable library so the math is auditable rather than hidden. Outputs are copyable for paste into a design note and the Length vs Baud chart prints for site documentation.
Inputs and outputs
What goes in, what comes out
Inputs 8
Serial standard (RS-232 / EIA-232, RS-422 / EIA-422, RS-485 / EIA-485, CAN Bus per ISO 11898, I²C Fast mode)
How is the maximum reliable cable length worked out?
Length mode solves the maximum length against three constraints at once: the bit time / propagation delay rule (cable propagation delay typically under one tenth of a bit time), the cable capacitance limit (TIA-232-F 2500 pF for RS-232, I²C 400 pF), and the standard length / baud product (TIA-422-B and TIA-485-A length-baud product, ISO 11898 CAN bit time × length product). The tightest of the three wins and is named alongside the safe operating length, so you know whether the link is capacitance limited, timing limited, or standard limited.
What does Integrity mode compute?
Integrity mode computes the RC rise time at the receiver from the driver source impedance, the cable capacitance, and the length (t_r ≈ 2.2 · R · C). It reports the resulting bandwidth (BW = 0.35 / t_r) and the reflection coefficient at the termination (Γ = (Z_t − Z₀)/(Z_t + Z₀)). The eye margin is computed against the bit period using the driver rise time and the driver output voltage as inputs, with a verdict that flags the rise time as adequate, marginal, or excessive.
How does the noise budget work?
Noise mode builds the cumulative differential noise budget at the receiver. The induced noise from adjacent cables is reduced by the CMRR of the differential receiver, the ground potential difference between the two ends is reduced by the shield attenuation, and the IR drop along the cable resistance at the load current is added. The final differential voltage is compared against the configured receiver sensitivity (typical 200 mV for RS-485, but configurable per part), and the link is flagged PASS, MARGINAL, or FAIL.
Which cables are in the library, and can I use my own values?
The library covers Cat5e / Cat6 UTP, shielded twisted pair, Belden 3105A / 9841 / 8132 RS-485 reference cables, low capacitance RS-232 cables, CAN cables, and I²C cables. Each carries datasheet anchored capacitance (pF/m), characteristic impedance (Z0), velocity factor (% c), and DC resistance (Ω/km). Any value can be overridden for a non standard cable or for measured data, so the result reflects the actual cable on site rather than the library default.
Which standards does the calculator check against?
TIA / EIA-232-F for RS-232 (including the 2500 pF capacitance limit), TIA / EIA-422-B for RS-422 and TIA / EIA-485-A for RS-485 (the length / baud product), ISO 11898 for CAN bus (the bit time × length product), and I²C UM10204 for Fast mode (including the 400 pF bus capacitance limit). The reference panel surfaces these limits side by side so the result can be cross checked against the published limit.
What is the difference between one way and round trip propagation delay?
Timing mode reports both. The one way delay is the cable length divided by the propagation velocity (set by the velocity factor). The round trip delay is the relevant figure for half duplex protocols where the master must wait for the slave reply before driving the line again. The timing margin between the propagation delay and the bit period is reported in nanoseconds and as a percentage of the bit time, so a borderline link is identified directly.
Does any data leave my browser?
No. All length, integrity, timing, and noise computation runs entirely in your browser, and no design data is uploaded to a server. That makes it suitable for OT / SCADA work under an air gap policy. Outputs are copyable to the clipboard for paste into a design note, and the Length vs Baud chart prints for site documentation.
