PROFIBUS and PROFINET are industrial networks that carry many devices on one segment. A device on either is a network node with an address rather than a wired point on an I/O card, and that changes how you count and list it.
This is a reference for the engineer taking a P&ID with networked instruments onto an I/O list. It covers what PROFIBUS and PROFINET are in plain words, how a networked device differs from a hardwired 4 to 20 mA point, device count versus channel count, and how to list networked devices without double-counting. It is not a network-design or device-configuration guide.
Key takeaways
- PROFIBUS is a fieldbus and PROFINET is industrial Ethernet. Both put many devices on one segment instead of one point per card channel.
- A networked device still needs a tag and still carries process data. It is a node with an address, not a wired channel.
- On the I/O list a networked device is a row keyed by tag, but its point is a data mapping, not a physical terminal.
- Count devices and the variables they expose, not card channels. One segment can carry devices that would each have taken a hardwired channel.
- Remote I/O is the exception: the rack is one node, but the instruments wired into it are still physical channels.
PROFIBUS and PROFINET in plain words
Both replace the one-signal-per-wire model with a shared digital network, and the difference between them is the medium. PROFIBUS is a fieldbus: a dedicated bus, one pair, with devices addressed along the segment. Its DP form is used for fast exchange with remote I/O and drives, and its PA form is used for process instruments, carrying the device power on the same pair and suiting hazardous areas. PROFINET is industrial Ethernet: the same many-devices-on-shared-media idea, but over standard Ethernet through switches, with each device a network node.
A plant often runs both. A common arrangement uses PROFINET as the backbone and hangs PROFIBUS PA segments of process instruments off it through a coupler, so the process devices sit on a bus suited to the field while the fast traffic runs on Ethernet.
Hardwired and networked, side by side
The shift from hardwired to networked is the shift from a channel to a node.
| Hardwired 4 to 20 mA | Networked device | |
|---|---|---|
| Connection | One pair to one card channel | One of many nodes on a shared segment |
| Identity | A terminal and channel number | A device address on the network |
| Data | One analog value per wire | A set of variables in the device's process image |
| Count basis | Card channels | Devices and their mapped variables |
On the P&ID a networked instrument can look the same as any other bubble. The fact that it is on a network usually shows on the network or system architecture diagram, not on the P&ID itself, so the I/O list and the architecture drawing have to be read together to know how a given instrument connects.
Device count versus channel count
The mistake a hardwired habit produces is counting a networked segment as if every signal were a card channel. It is not. A native fieldbus or PROFINET transmitter exposes its primary variable, its status, and often secondary variables, all as data in the network, and none of them consume a physical channel. The controller reads them over the segment.
Remote I/O is where the count gets subtle, because it is a hybrid. A remote I/O rack is a single node on the network, but the instruments wired into its cards are ordinary hardwired points that happen to terminate in a remote location. Those instruments are still physical channels: a hardwired level switch on a card in a remote rack is a digital input, whether the rack is next to the controller or across the plant. So a segment can carry native devices that are nodes and remote racks that are nodes but that themselves contain channels. Getting the count right means telling the two apart.
A worked example
The tags below are synthetic. This is one PROFINET segment off a controller PLC-1, carrying three native field devices and one remote I/O rack.
| Tag | Connection | On the I/O list |
|---|---|---|
FT-101 | Native PROFINET device, node on the segment | One device, mapped variables, no card channel |
PT-101 | Native PROFINET device, node on the segment | One device, mapped variables, no card channel |
TT-104 | Native PROFINET device, node on the segment | One device, mapped variables, no card channel |
LSL-402 | Hardwired to a card in a remote I/O rack | One DI channel, in the remote rack node |
XV-301 | Hardwired to a card in a remote I/O rack | One DO channel, in the remote rack node |
The three native devices are three nodes and zero card channels. The remote rack is one node, but LSL-402 and XV-301 are still a hardwired digital input and digital output inside it. Counting the segment as five card channels would be wrong, and counting it as five nodes would be wrong too. It is three native nodes plus one rack node that contains two channels.
How to list a networked device
A networked device keeps the same tag it has everywhere else, so the I/O list stays keyed by tag. What changes is the location and point columns.
| Column | Hardwired point | Networked device |
|---|---|---|
| Tag | Instrument tag | Instrument tag |
| Signal class | AI, AO, DI, DO | The class of each mapped variable, marked as networked |
| Location | Rack, card, channel | Segment and device address |
| Point | Physical terminal | Data map in the device process image |
| Consumes | One card channel | No card channel; controller address space |
The soft versus hard I/O distinction is what keeps this straight: a networked variable is soft, mapped in the controller, while a hardwired point is hard, terminated on a card. Marking each device as networked or hardwired on the I/O list is what stops a smart transmitter from being counted twice, once as an analog input and again as a network node.
How it reconciles with the rest of the set
The networked devices tie three documents together. Every native device is a node on the network or segment list and a data map in the controller, and it appears on the I/O list with no card channel. Every hardwired point behind a remote rack is a channel that still counts toward card and rack sizing, even though the rack is remote. Reconciling the I/O list against the network architecture is what catches a device counted as a hardwired channel it never used, or a remote-rack instrument dropped from the channel count because the rack was treated as a single node.
A starting point
If you are listing networked instruments, the PROFINET reference and the PROFIBUS reference cover the two networks, and the 4 to 20 mA, HART, and fieldbus reference covers how a hardwired signal differs from a networked one. Mark every device as networked or hardwired from the first draft of the I/O list: it is the field that decides whether a device consumes a card channel, and it is the one that keeps a fieldbus segment from being either double-counted or dropped.