Choose a pneumatic valve block by its electrical interface first: coils your PLC output points can drive at their rated voltage and current, suppression for the inductive kick, and status inputs fed back over the same connection. Count valves plus spare points after that; the 8 or 16 point decision comes last, after the interface matches.
A pneumatic valve block becomes remote I/O the moment its electrical interface matches the PLC: coils the output points can drive at their rated voltage and current, suppression for the inductive kick, and status inputs the controller can read back. Pick the block on those checks first. The 8 versus 16 point question is sizing, and it only means something once the interface matches. This article runs the checks, compares the block types, and works a 16-valve example through to the supply size.
What makes a valve block remote I/O
The PLC has to do two things to the block: switch each valve as an output point, and read back as inputs whatever confirmation the machine needs. Two routes get you there.
Discrete wiring gives each coil its own wire pair back to an output card. The PLC sees one point per valve, there is nothing to configure, and the card datasheet alone decides whether it can carry the coil.
A fieldbus node puts the connection on the plant network. The manifold carries an adapter for EtherNet/IP, Modbus TCP or PROFINET, and one cable carries every output, every sensor input and the diagnostics. The PLC scans the node like any other remote I/O drop.
The station or relay count is a sizing number. The interface decides whether the PLC sees the block at all, so read it off the block datasheet before anything else.
Match the coil to the output point
Two numbers have to agree: voltage and current.
Voltage first. A transistor output rated 24 V DC drives coils of that same voltage. A relay output card switches AC or DC within its contact rating. Never feed the coil from a higher voltage: coil current follows Ohm's law, so 48 V across a 24 V coil doubles the current and heats the coil four times faster until it burns. Solenoid manufacturers print voltage and watt ratings on coils for exactly this reason.
Current next. Divide coil watts by coil volts. Small valve solenoids at that voltage typically draw 2 to 8 W, which is 0.08 to 0.33 A per coil, and output points on small transistor cards are commonly rated about 0.5 A. A coil at or under 0.3 A sits well inside a 0.5 A point. The manifold datasheet and the card manual print the exact figures, and those figures decide it.
Last, suppression. A solenoid coil is an inductive load, and opening the point spikes the voltage as the field collapses. Cards with built-in suppression list it in their manual. Where the card has none, fit a suppressor at the coil; the FAQ below covers the type and rating.
Count the feedback inputs, then the points
Decide which stations need position proof. A double-acting valve takes one reed or hall sensor per position, so full confirmation on a 16-station block means 32 input points. Discrete wiring sends those to a separate input card; many fieldbus blocks bring sensor inputs into the same node, so one cable carries both directions.
Then count outputs: stations in use plus 20% spare, the usual panel-design target. Eight stations plus spare fits an eight-point card or a small node. Sixteen stations plus spare needs 20 points, which a single 16-point card misses. That arithmetic, run after the interface checks pass, is the real 8 versus 16 decision.
The block types compared
| Block type | How the PLC sees it | Coil power | Status inputs | Choose it when |
|---|---|---|---|---|
| Discrete-wired valve manifold | One output point per coil on an output card | External supply sized for the coil count | Sensors wire to a separate input card | Few stations and spare card points; simplest to fault-find |
| Fieldbus valve manifold (EtherNet/IP, Modbus TCP, PROFINET) | A bank of remote I/O points over one network cable | The block manual states its supply arrangement | Sensor inputs join the same node on many blocks | Sixteen or more stations, position feedback, or one cable across the machine |
| Relay terminal block feeding separate valves | One output point per relay contact | Contacts switch the coil voltage within their contact rating | None inherent; add sensors as discrete inputs | Mixed AC and DC coils, or isolating the card from the coil load |
| Modular valve system with an adapter interface | Stations appear through an adapter card or a discrete-control supply module | The adapter or supply module sets the electrical limits | Depends on the adapter | You are standardizing one modular valve family across machines |
At least one fieldbus manifold family carries a small graphical display at the block for configuration, commissioning and diagnostics, so a technician can cycle a station from the manifold without opening the PLC program.
Example: sizing a 16-valve block
Inputs, and where each comes from:
- 16 valve stations: the larger of the two common block sizes.
- 5 W per coil at 24 V DC: mid-range of the 2 to 8 W typical for small valve solenoids; your manifold datasheet prints the real figure.
- 0.5 A per point: the common rating on small transistor output cards; your card manual prints the real figure.
- 20% spare points and a 1.5 times supply margin: the usual panel-design targets.
The rule in words: per-coil current equals coil watts divided by coil volts. Block load equals per-coil current times the stations that energize together. A point fits when per-coil current sits inside the point rating with margin. Supply size equals the all-on load times the margin.
The arithmetic:
- Per coil: 5 W ÷ 24 V = 0.21 A.
- All 16 on together: 16 × 0.21 A = 3.3 A.
- Point check: 0.21 A is 42% of a 0.5 A point, so every valve fits with margin.
- Outputs with spare: 16 × 1.2 = 19.2, so plan 20 output points; a single 16-point card is short, so take a 32-point node or a 16-point card plus an 8-point card.
- Feedback: 2 sensors × 16 stations = 32 inputs, which the fieldbus node carries on the same cable.
- Supply: 3.3 A × 1.5 = 5 A, so a 5 A supply at the coil voltage runs all 16 valves with margin.
The decision: at 5 W per coil the block fits a standard 0.5 A card and a 5 A supply. The same card stops fitting at 12 W per coil, because 0.5 A × 24 V = 12 W; above that threshold, fit an interposing relay per valve or pick a block that feeds its own coils. Total every other load sharing the valve rail before you buy the supply, and commission with the actuators vented or the machine in a safe state: drive each point, watch the spool indicator, then reconnect the loads.
Keep the coils off the panel electronics supply
Solenoid coils dump switching noise into whatever supply they share. The recurring panel failure is one supply rail feeding both the coils and the operator interface: the screen freezes or resets every time a bank of valves drops out. Read why a shared DC supply with solenoid coils freezes an HMI for the symptom pattern, and sizing a separate supply for HMI inrush current walks the margin math when the coil bank gets its own rail. Separate rails or a second supply, and tie the commons the way the card manual's wiring diagram shows.
When to pick something else
- AC coils already on the machine and no AC relay card: fit interposing relays, and let the transistor points switch the relay coils.
- Two to four valves and spare card points: wire discrete; a fieldbus node adds cost and configuration this machine does not need.
- Every station needs position proof: take a fieldbus block with sensor inputs, sized per the example.
- Coils above 12 W: relay contacts or a block with its own coil supply, per the example threshold.
- The floor also needs to watch the states: that is a separate screen decision, and factory status display or operator HMI explains the split.
Recommendation: wire a discrete manifold at the coil voltage when the machine carries eight or fewer valves and the output card has spare points; choose an EtherNet/IP or Modbus TCP fieldbus block when stations reach sixteen, when position inputs ride along, or when one cable across the machine is worth the adapter. In both cases keep each coil at or under half the point rating, fit suppression where the card lacks it, and give the coils their own supply sized at 1.5 times the all-on load.
What to ask for before you order
- From the manifold datasheet: coil voltage, watts per station, suppression arrangement, and how many sensor inputs the block accepts.
- From the PLC manual: output card type, transistor or relay, per-point current rating, and whether built-in suppression is listed.
- From the network side: which protocol the block's adapter speaks and what node type it presents in the PLC configuration.
- Behavior, in writing: what the outputs do on power loss and on communications loss, from the block manual.
- Your own arithmetic: stations plus 20% spare, sensor count, and the all-on load in amps that sizes the supply.
Questions
Can a 24 V DC transistor output drive a 110 V AC valve coil?
No. The point and the coil must agree on voltage and on AC or DC. Drive the AC coil from a relay output card whose contacts are rated for that inductive AC load, or put an interposing relay between them and let the transistor point switch the relay coil. Check the contact rating for inductive loads, not just resistive, before wiring.
What do the valves do when the network cable is pulled?
A spring-return valve goes to its rest position whenever the coil de-energizes, so power loss means home position for every station. A fieldbus block's manual states separately what its outputs do when communications drop; build that case into the bench test before the machine runs production. If the sequence cannot tolerate a drop to home, hold the needed states with latched logic in the PLC.
What kind of suppressor goes across the coil?
Where the output card has no built-in suppression, the I/O module documentation covers a universal surge suppressor rated for coils up to 150 V AC or DC, fitted across the coil as the module manual shows. It clamps the inductive kick that would otherwise pit relay contacts or punch through an output transistor.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.