ControlNet network sizing
A ControlNet link is a tree of linear coax segments joined by repeaters. Each segment is limited by the number of taps on it, the link by the number of repeaters in series and by the propagation delay between the two farthest nodes, and the schedule by NUT, RPI and the 500 bytes a node may send per NUT. This page shows the rules of the Rockwell planning manuals the way the calculator applies them.
When to use
When laying out ControlNet coax trunks and deciding where a repeater is needed, when extending a plant link with a fiber repeater pair or an optical ring, when migrating or auditing an existing ControlLogix network, and before setting NUT, SMAX and UMAX in RSNetWorx for ControlNet.
How a ControlNet link is built
The coax segment is strictly linear — trunk with taps, a 75 Ω terminator at each end — and every node hangs on its own tap with a 1 m drop that cannot be cut or lengthened. Branching is only legitimate at a repeater: a 1786-RPCD opens new coax segments, and a pair of fiber modules carries the link to a remote area, point to point or in an optical ring. The calculator partitions the tree into segments and checks each one, then checks the link as a whole. In the same plants, device-level I/O often hangs below ControlNet on DeviceNet, and in Siemens-based plants the equivalent cell-level role goes to PROFIBUS DP.
The segment: taps, cable and length
Every tap on a segment costs 16.3 m of the 1000 m base: nodes, repeaters, spare taps with a test load, the maintenance tap and reserved bullet connectors. Up to 48 taps, which leaves 250 m. Cable other than standard RG-6 enters by equivalent length, in the ratio of its attenuation: RG6F weighs 2.25 times, a jumper about twice. The DS/3/4 drop cable used as trunk has a hard cap of 30 m per segment, and plug-to-plug jumpers 70 per segment. Measure the trunk along its real route in the cable trays. Unlike DeviceNet or a PROFIBUS PA segment, the coax carries no power, so length is the only cable budget — there is no voltage drop to check.
Repeaters, fiber and delay
Repeaters in series are capped at 20 with the B-series adapter and 5 with the A series; each adapter takes up to 4 modules and 1.6 A of backplane current. The propagation delay is summed over coax, fiber and repeaters and must stay under 121 µs per direction between any two nodes — the calculator finds the worst pair, not just the longest trunk. Each fiber link has its own optical budget and range by module, and an optical ring is checked including its closing span and the degraded case after a break. With redundant media the A/B skew is limited to 1.6 µs. Where the fiber links end in patch panels in the control room, their space is sized in 19” rack sizing.
Addresses and scheduled data
Addresses run from 1 to 99, unique per link. SMAX must cover the highest scheduled node and UMAX the highest node in use, without leaving empty addresses behind. For scheduled data the API comes from the RPI as the NUT times a power of 2, each node may send at most 500 bytes per NUT, and the connections of each communication module are checked against the published limits (1756-CN2, 1756-CNB, 1768-CNB, 1769-L3xC, 1784-PCICS, 1788-CNx) or a declared limit.
What this tool does not cover
It does not estimate the percentage of scheduled bandwidth or produce the schedule — that is RSNetWorx for ControlNet’s job, and the frame overhead has no primary published source. Fiber attenuation is not assumed: without it the optical margin is not evaluated. Unpublished cable attenuation (DS/3/4 drop cable) is handled by its 30 m cap, not by equivalence. And above 48 taps the length line is not extrapolated.
Formulas and fundamentals
sum_i (a_i / 5.99) × L_i <= 1000 − 16.3 × (N_taps − 2) [m] The three criteria Rockwell publishes (length table, attenuation form and equivalent-length example of 1786-IN009B) are the same rule. The left side is the RG-6 equivalent length: RG6F (13.5 dB/304 m) counts 2.25 times its length, and a 1786-TJPR jumper about twice. Above 48 taps there is no extrapolation.
delay = sum(L_coax × 4.17 ns/m) + sum(L_fiber × 5.01 ns/m) + sum(901 ns + module delay) <= 121 µs Per direction, between ANY two nodes — the calculator scans every pair for the worst one. Each repeater costs the 901 ns of the 1786-RPA/B adapter plus its module (100 ns RPCD, 94 RPFS, 153 RPFM, 100 RPFRL/RPFRXL). The route is also capped at 20 km, checked separately.
skew = 4.17 × dL_coax + 5.01 × dL_fiber <= 1.6 µs Difference between channels A and B. Both channels must also cross the same number and type of repeaters, and redundant and non-redundant nodes may not be mixed in the same link.
margin = budget − (alpha × L + n_conn × a_conn + n_splice × a_splice) >= 0 Budget per module: RPFS 4.2 dB (with 2 dB charged per coupled pair), RPFM 13.3 dB, RPFRL/B 15 dB, RPFRXL/B 10.5 dB. Fiber attenuation is an input — without it the margin is not evaluated. The RPFRL/B does not accept single-mode fiber.
API = NUT × 2^k <= RPI, k <= 7; bytes per node per NUT <= 500 NUT from 2 to 100 ms; RPI must not be faster than the NUT. A rack-optimized connection adds 8 bytes per chassis slot. A produced or consumed tag is limited to 480 bytes.
Standards & methods
- Rockwell CNET-IN002B — ControlNet Coax Media Planning and Installation Guide
- Rockwell CNET-IN001C — ControlNet Fiber Media Planning and Installation Guide
- Rockwell CNET-UM001H — ControlNet Network Configuration
- Rockwell 1786-TD008A — ControlNet Accessory Specifications
- Rockwell 1786-IN009B — ControlNet Standard and High-flex Coax Cable
- Rockwell 1786-IN007 — ControlNet Coax Taps
- ODVA — ControlNet specification
Typical reference values
| Quantity | Typical range | Note |
|---|---|---|
| Segment length in RG-6 | 1000 m (2 taps) · 934.8 m (6 taps) · 250 m (48 taps) | — |
| Taps per segment | ≤ 48 | a repeater takes one tap on each segment it touches |
| Cable attenuation at 10 MHz | RG6 5.99 · RG6F 13.5 dB per 304 m | — |
| Jumpers / DS/3/4 drop cable as trunk | ≤ 70 jumpers · ≤ 30 m per segment | — |
| Nodes and addresses per link | 99 nodes, addresses 1–99 | 98 with an optical ring module |
| Repeaters in series | 20 with 1786-RPA/B · 5 with 1786-RPA/A | — |
| 1786-RPA/B adapter | 4 modules · 1.6 A backplane at 5 V | — |
| Delay / route / skew | 121 µs per direction · 20 km · 1.6 µs | — |
| NUT / RPI | 2–100 ms / 2–750 ms | — |
Worked example
One RG-6 segment, five nodes and a maintenance tap
Inputs
- Taps along the trunk (distance from the previous)
- PLC 0 · RIO-01 180 · RIO-02 220 · VFD-01 260 · HMI-01 150 · maintenance 40 m
- Cable / NUT / adapter series
- 1786-RG6 · 5 ms · B —
- Scheduled RPI (PLC · RIO-01 · RIO-02 · VFD-01)
- 10 · 20 · 20 · 40 ms
- RIO-01 rack-optimized connection
- 32 in + 32 out + 10 slots bytes / slots
- SMAX / UMAX declared
- 8 / 8 —
Results
- Trunk length (RG-6 equivalent)
- 850 m
- Segment limit with 6 taps
- 934.8 m (91 % used)
- Worst propagation delay (PLC ↔ HMI-01)
- 3.38 µs (limit 121 µs)
- API (PLC · RIO · VFD)
- 10 · 20 · 40 ms
- Largest scheduled data per node (RIO-01)
- 144 bytes per NUT (limit 500)
- Suggested SMAX / UMAX
- 7 / 5 —
- Verdict
- OK —
Six taps — five nodes plus a maintenance tap — bring the segment limit down to 1000 − 16.3 × 4 = 934.8 m, and the 850 m of trunk use 91 % of it: one more node and a few tens of metres would require a repeater. The delay is not a concern on a single segment: 810 m between the two farthest nodes give 3.4 µs of the 121 µs. The RPIs fall exactly on binary multiples of the 5 ms NUT. The declared UMAX of 8 is loose — addresses 6, 7 and 8 are empty and still get a turn —, so the calculator suggests UMAX 5 and SMAX 7 (highest scheduled address 4 plus 3 for growth).
Common mistakes
- Counting only nodes as taps. Spare taps with a TCAP, maintenance taps, reserved bullet connectors and repeaters all take a tap and cost 16.3 m each.
- Treating RG6F as RG6. High-flex cable has 13.5 dB per 304 m against 5.99: 100 m of RG6F use 225 m of the segment budget.
- Checking delay only along the longest trunk. The worst pair may be elsewhere: each repeater weighs about 1 µs of delay — some 240 m of coax — and zero metres of route.
- Assuming the 1786-RPA/A behaves like the B series. The A series allows only 5 repeaters in series, not 20. Without the series declared the calculator does not pick one for you.
- Leaving UMAX far above the highest address. Every address up to UMAX gets an unscheduled turn; empty ones waste bandwidth. And a node above UMAX is on the cable but never polled.
- Setting SMAX below the highest scheduled address. That node cannot send scheduled data; with an optical ring module SMAX also needs one free address above it.
Frequently asked questions
What is the maximum length of a ControlNet segment?
1000 m of RG-6 with 2 taps, falling 16.3 m for every additional tap down to 250 m with 48 taps. Beyond that, a repeater opens a new segment. With other cables the limit applies to the RG-6 equivalent length.
How many nodes can a ControlNet network have?
99 nodes per link, addresses 1 to 99, and up to 48 taps per segment. Repeaters take taps but do not take addresses. With an optical ring module the highest usable address falls to 98.
How many repeaters can be connected in series?
20 with the 1786-RPA/B adapter and 5 with the 1786-RPA/A. In addition, the total propagation delay between any two nodes must stay under 121 µs, and each adapter is limited to 4 modules and 1.6 A of backplane current — whichever is reached first.
Does the calculator estimate scheduled bandwidth?
No. Scheduling is done by RSNetWorx for ControlNet, and the link-layer frame overhead has no primary published source. The calculator checks NUT, RPI, API, bytes per node per NUT, tag size and connections per module, and leaves the bandwidth percentage to RSNetWorx.
How are SMAX and UMAX chosen?
SMAX must be at least the highest scheduled address — plus one with an optical ring module — and the usual practice is to leave about 3 addresses of room. UMAX must not be below SMAX nor below the highest address in use, and should not be much above it.
Does ControlNet coax carry power?
No. There is no supply, voltage drop or current budget on the media. The only current budget is the 1.6 A at 5 V of the repeater adapter backplane.
Glossary
- Segment
- Linear coax trunk with taps, terminated by a 75 Ω terminator at each end.
- Link
- Set of segments joined by repeaters; between two points there is a single path.
- Tap
- Passive connection to the trunk with a fixed 1 m drop; every node and every repeater takes one.
- NUT
- Network Update Time — the repeating interval in which scheduled and unscheduled traffic must fit.
- RPI / API
- Requested and actual packet interval; the API is the NUT times a power of 2 not slower than the RPI.
- SMAX / UMAX
- Highest address allowed to send scheduled data / highest address that gets an unscheduled turn.
- Keeper
- Node that holds and distributes the network schedule parameters.