Automation

DeviceNet network sizing

A DeviceNet network is limited by four things at once — trunk length for the data rate, the length and current of every drop line, the voltage left at the farthest node, and the current the cable and the supply can carry. This page shows the rules of the ODVA cable system manual (PUB00027R1) the way the calculator applies them, with a worked example from chapter 4.

When to use

When laying out a new DeviceNet trunk or extending an existing one, when deciding where the 24 V supply goes and whether a second power tap is needed, when choosing between thick, mid, thin and flat cable, and before raising the data rate of a network that already works at 125 kbit/s.

What limits a DeviceNet network

DeviceNet carries data and 24 V power on the same cable — like a PROFIBUS PA segment and unlike PROFIBUS DP or ControlNet, whose media carry no power — so the trunk is limited by two independent physics. The CAN signal limits length: the faster the data rate, the shorter the trunk and the smaller the total of drop lines. The 24 V supply limits current and voltage: every section of trunk drops I × R in V+ and again in V−, and the farthest device must still see at least 11 V. A network can pass one check with room to spare and fail the other.

Length, drops and data rate

The length limit applies to the trunk, measured terminator to terminator. Each cable type has its own limit per data rate, and in a mixed trunk each section consumes a fraction of the limit of its own cable. Use the routed lengths along the cable trays, not the distance on the plot plan. Drop lines are capped at 6 m each and have a cumulative budget that shrinks as the rate rises. The calculator redoes the count at 500, 250 and 125 kbit/s and reports the highest rate the topology allows — useful before deciding to speed up a network.

Voltage drop and supply placement

The drop is calculated section by section with the current that actually crosses each one, plus 0.005 Ω of contact per tap. That is the same identity as the chapter 4 formula of the ODVA manual, exact even when the trunk mixes cables — the bus version of the round-trip drop used in DC cable sizing. A supply placed off the end creates two sections; a power tap with V+ interrupted creates separate power domains, and each needs its own supply. Where the network must stay up through an outage, that 24 V supply is backed by a battery bank. Each drop line is also checked against its own current limit, 4.57 / L amperes.

Topology and addressing

The DeviceNet trunk is a line, whether it runs past sensors and valve islands or through the starters of the MCC drawers. A branch in the trunk breaks impedance matching and fails the network; terminators are deduced at the two ends. Devices hung directly on a power tap or on the trunk end are flagged as orphans. Addresses go from 0 to 63, with up to 64 nodes; duplicated or out-of-range MAC IDs fail, and 63 — the address new devices come with — raises a warning.

What this tool does not cover

It does not estimate bus load or scan time: the input and output bytes are only added up. It does not solve two supplies in parallel exactly — it takes the worse of each one feeding the stretch alone, which overestimates the drop. Grounding is a reminder (V−, drain and shield bonded at a single point), not a calculation. And it does not replace the voltage and current data of the device datasheets: the device current is an input, never a default.

Formulas and fundamentals

Voltage drop per section dV(node) = sum over sections [ I_crossing × (l × Rc + R_tap) ], R_tap = 0.005 ohm

The same identity as the ODVA chapter 4 formula sum{[(Ln·Rc) + (Nt·0.005)]·In}, regrouped by section instead of by device. On a homogeneous trunk both give the same number; on a mixed trunk the per-section form is the exact one, and the calculator also shows the manual's single-Rc value for cross-checking. The power tap does not count as a tap.

Node voltage V_node = V_supply,min − 2 × (dV_trunk + I_device × L_drop × Rc_drop) with 11 V <= V_node <= 25 V

Drop in V+ and in V−, hence the factor 2. V_supply,min = 24 V × (1 − 3.25 %) = 23.22 V by default. The minimum is 11 V, or 19.2 V for devices whose outputs are powered from the network.

Length budget for mixed cable sum( L_cable / L_max,cable(rate) ) <= 1

Each trunk section is charged against the limit of its own cable at the chosen rate. At 125 kbit/s this is equivalent to L_thick + 5·L_thin <= 500 m. ODVA does not write a closed mixing rule; this is the modelling choice that reproduces the ratio of the tabulated limits.

Drop line current I_drop <= min( 3 A ; 4.57 / L_drop ; cable capacity )

L_drop in metres, up to 6 m. A drop of 2 m takes up to 2.29 A; one of 6 m, 0.76 A. Direct connection counts as a drop of zero length.

Supply current I_supply,nominal >= 1.10 × sum(I_devices)

Sum of the device currents in the section plus 10 % for inrush, against the rating declared for the supply.

Standards & methods

ODVA PUB00027R1 — DeviceNet Cable System Planning and Installation Manual (ch. 1, ch. 4, appendices A and B)
ODVA CIP Networks Library Vol. 3 — DeviceNet Adaptation of CIP
NFPA 70 (NEC) — Class 1 and Class 2 circuits

Typical reference values

Quantity	Typical range	Note
Maximum trunk, thick cable	500 m @ 125 · 250 m @ 250 · 100 m @ 500 kbit/s	—
Maximum trunk, flat cable	420 m @ 125 · 200 m @ 250 · 75 m @ 500 kbit/s	—
Maximum trunk, mid / thin cable	mid 300 / 250 / 100 m · thin 100 m at any rate	—
Drop line	≤ 6 m each; cumulative 156 / 78 / 39 m	at 125 / 250 / 500 kbit/s
Cable resistance Rc	thick 0.015 · mid 0.023 · thin 0.069 · flat 0.019 Ω/m	—
Cable current capacity	thick 8 A · flat 8 A · thin 3 A · mid 1.5 A	NEC Class 2 caps everything at 4 A
Nodes and addresses	up to 64 nodes, MAC ID 0–63	63 is the default address of new devices — avoid it
Node supply range	11 to 25 V DC	—

Worked example

Supply at the end, thick trunk, four devices (ODVA PUB00027R1, ch. 4, example 1)

Inputs

Data rate / NEC class: 125 kbit/s · Class 1 —
Supply PT-01 at the trunk end: 24 V − 3.25 % · 4 A —
Taps along the thick trunk: 15 · 30 · 122 · 244 m
Device currents D-01 … D-04: 1.0 · 0.5 · 0.5 · 0.25 A
Thin drop lines: 2 · 2 · 3 · 3 m

Results

Trunk length / length budget used: 244 m · 0.488 of 500 m
Trunk voltage drop per conductor: 2.30 V (limit 4.65 V, 50 %)
Lowest node voltage (D-04): 18.51 V (minimum 11 V)
Supply current with 10 % inrush: 2.48 A (rating 4 A)
Cumulative drop length: 10 m (limit 156 m)
Highest feasible data rate: 250 kbit/s
Verdict: OK —

The 2.25 A of the four devices cross the first 15 m; only 0.25 A reach the last 122 m section. Summing I × (l·0.015 + 0.005) section by section gives 2.30 V, half of the 4.65 V budget, and the farthest device still sees 23.22 − 2 × 2.35 = 18.51 V. The network would also run at 250 kbit/s (244 m of 250 m, 10 m of the 78 m drop budget), though with a length budget at 98 % — little room for an extension. At 500 kbit/s the 100 m thick limit rules it out.

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Common mistakes

Checking length and forgetting voltage. A 240 m thick trunk is well inside 500 m at 125 kbit/s, but about 1.3 A drawn at its far end already use up the whole 4.65 V budget (240 m × 0.015 Ω/m × 1.3 A).
Measuring voltage drop on one conductor and comparing node voltage to 11 V. The current goes out on V+ and back on V−: the node sees the supply minus twice the drop.
Using a 6 m drop for a 1 A device. The drop current limit is 4.57 / L — at 6 m only 0.76 A pass. Shorten the drop or connect the device directly.
Placing the supply at one end of a long trunk. Moving it to the centre creates two sections that are checked independently, and each carries half the current over half the length.
Raising the data rate without redoing the count. At 500 kbit/s the thick trunk drops to 100 m and the cumulative drop budget to 39 m.
Leaving a new device at MAC ID 63. It is the out-of-box address; the second one to arrive collides with it.

Frequently asked questions

What is the maximum length of a DeviceNet network?

With thick cable, 500 m at 125 kbit/s, 250 m at 250 kbit/s and 100 m at 500 kbit/s. Thin cable is limited to 100 m at any rate. With mixed cable each section uses up its fraction of its own cable limit, and the fractions must add up to at most 1. The calculator applies the limit to the trunk; the distance between the two farthest nodes, drops included, is reported as a warning.

How long can a DeviceNet drop line be?

6 m each, at any data rate, with a cumulative total of 156 m at 125 kbit/s, 78 m at 250 kbit/s and 39 m at 500 kbit/s. The current is also limited: 4.57 / L amperes, never more than 3 A.

Where should the DeviceNet power supply go?

Usually near the centre of the load. A supply in the middle of the trunk splits it into two sections checked separately. With more than one supply, a power tap with V+ interrupted isolates the upstream side, which then needs its own supply; two supplies in parallel without the cut are evaluated as if each fed the stretch alone, which is an upper bound for the real drop.

Why does the node voltage use twice the voltage drop?

Because current flows out on V+ and back on V−. The 4.65 V budget is per conductor, and the node sees the minimum supply voltage minus the drop on both.

Why 23.22 V and not 24 V?

Chapter 1 of PUB00027R1 itemises a 3.25 % stack-up for the supply (setting, line and load regulation, drift). Appendix B uses 23.00 V (4 %) instead; if your criterion is the stricter one, declare the tolerance on the supply.

Is the device current mandatory?

Yes. There is no typical consumption per device family in DeviceNet, and without current the voltage-drop check cannot be done: the network comes out "not evaluated", not "OK".

Glossary

Trunk line: Main cable of the network, linear, with a terminating resistor at each end.
Drop line: Branch from a tap on the trunk to the device, up to 6 m.
Power tap: Trunk tap that connects the 24 V supply; it can interrupt V+ to split the trunk into power domains.
MAC ID: Node address on DeviceNet, from 0 to 63; it must be unique on the network.
Thick / mid / thin / flat cable: DeviceNet cable types, with different resistance, current capacity and length limit.
NEC Class 2: Power-limited circuit category; caps the network current at 4 A regardless of cable.