How Many Modbus Networks Can a JACE Support?
A JACE runs one Modbus RTU network per RS-485 port, so capacity depends on onboard ports plus expansion modules, while Modbus TCP devices share IP networks.
When a project has hundreds of Modbus meters, drives and plant controllers, the number of Modbus networks a controller can run decides how the system performs. This guide explains how Modbus networks work in Niagara, how many a JACE 8000, JACE 9000 or IONA can host, the difference between RTU and TCP networks, and how to design a Modbus integration that stays responsive.
What is a "Modbus network" in Niagara?
In a Niagara station, each Modbus integration is a network component under the Drivers folder:
- Modbus async (RTU/ASCII) network: one per serial port. It owns one RS-485 port and polls the devices on that trunk. Because RS-485 is a single shared bus with one master, you cannot put two Modbus RTU networks on the same port.
- Modbus TCP network: talks to Modbus TCP devices over Ethernet. One TCP network can contain many devices, each identified by IP address and unit ID.
- Modbus TCP gateway: a TCP network configuration used to reach serial devices behind a Modbus TCP to RTU gateway, addressing each by unit ID.
Each network has its own poll scheduler, so separate networks are polled in parallel. That is the main reason to split devices across more networks.
Modbus RTU networks per controller
The limit for Modbus RTU is the number of RS-485 ports:
| Controller | Onboard RS-485 | Expansion | Typical maximum Modbus RTU networks |
|---|---|---|---|
| JACE 9000 | 2 (isolated) | Up to 4 option modules, maximum 2 RS-485 modules | Onboard ports plus RS-485 module ports |
| JACE 8000 | 2 | Option modules, including the dual RS-485 module | Onboard ports plus RS-485 module ports |
| IONA Niagara 4 controller | 2 general purpose + 1 dedicated eBMS/IO port | — | 2 for third-party Modbus devices, plus the eBMS/IO expansion bus |
So a JACE 9000 with two dual RS-485 modules can host six RS-485 networks: two onboard plus four on the modules. Remember that each port can carry only one protocol: a port used for BACnet MS/TP cannot also run Modbus RTU.
Check current option module specifications before ordering, as module types and port counts can change.
Modbus TCP networks
Modbus TCP is not limited by physical ports. A single Modbus TCP network can hold many devices, and you can add more TCP networks if you want to separate groups of devices (for example by building or by device type) so they are scheduled independently.
Practical limits come from:
- licence capacity: every Modbus proxy point and device counts towards Global Capacity (see Global Capacity points)
- controller resources: CPU and memory used by polling many devices quickly
- device limits: many Modbus TCP devices accept only a few simultaneous connections
Modbus TCP gateways: more serial networks over IP
When a site has more RS-485 trunks than the controller has ports, or the trunks are far from the controller, use Modbus TCP to RTU gateways:
- each gateway terminates one or more RS-485 trunks near the devices
- the controller reaches them over Ethernet with a Modbus TCP gateway network
- each serial device is addressed by the gateway's IP and its own unit ID
This keeps RS-485 runs short, avoids long cables back to the plant room, and adds as many serial segments as needed. Products such as the eBMS/IO TCP-CON gateway and other units in our integration gateway range do this job.
Be aware that a gateway's serial side is still one RS-485 bus with its own speed limits; spreading devices across several gateways improves performance more than putting everything behind one.
How to decide how many networks you need
- List all Modbus devices with their protocol (RTU or TCP), location and the points needed.
- Group RTU devices by location to keep trunks short and wiring simple.
- Estimate polling load: a 9,600-baud trunk can only complete a limited number of reads per second. As a rough guide, keep trunks to a few dozen devices, fewer if each has many points or needs fast updates.
- Separate fast and slow devices: put plant controllers needing responsive control on different trunks from meters read every few minutes.
- Allocate ports: onboard RS-485 first, then option modules, then TCP gateways for remote or additional trunks.
- Keep spare capacity: one free port or space on trunks for future additions.
See how many devices a Modbus network supports for detailed trunk sizing.
Example design
A plant room JACE 9000 must integrate:
- 12 boiler and pump Modbus RTU devices in the plant room
- 40 Modbus RTU energy meters spread across five floors
- 3 Modbus TCP chillers on the roof
- a BACnet MS/TP trunk of 30 VAV controllers
Design:
| Port / network | Devices |
|---|---|
| Onboard RS-485 port 1 | BACnet MS/TP: 30 VAV controllers |
| Onboard RS-485 port 2 | Modbus RTU: 12 plant devices (fast control) |
| Floors 1–5 | Modbus TCP gateways, one per floor riser, each with 8 meters |
| Ethernet | Modbus TCP network: 3 chillers; Modbus TCP gateway network: 5 floor gateways |
The plant trunk stays fast and short, meters are polled independently through the gateways without long serial runs, and an RS-485 option module slot remains free for future use.
Modbus RTU vs Modbus TCP: which to use for new devices
When a device offers both, choose based on where it is and how it is used:
| Consideration | Prefer Modbus RTU | Prefer Modbus TCP |
|---|---|---|
| Location | Close to other serial devices and the controller | Already on, or near, the IP network |
| Speed needed | Modest | Fast updates, many points |
| Cabling | Cheap twisted pair daisy chain | Network cable and a switch port |
| IT involvement | None | IP addresses, VLANs, possibly approval |
| Number of clients | One master only | Several systems can read the same device |
| Fault isolation | One bad device can disturb the trunk | Devices are independent |
Modbus TCP is often the better choice for large plant (chillers, CHP, large heat pumps) and for meters in comms rooms. RTU remains cheapest for many small devices in one area.
Licensing and resource considerations
More networks and devices are only useful if the controller is licensed and resourced for them:
- Global Capacity: every Modbus proxy point counts, and on some licences every device. Size the licence for all networks together.
- CPU and memory: many networks polling quickly use more processing; check the Resource Manager and keep headroom.
- Third-party Modbus drivers: some advanced Modbus features are provided by third-party drivers that need their own licences.
On large integrations, distributing Modbus networks across several controllers can be better than concentrating everything on one, both for performance and resilience.
IONA and Modbus
IONA's two general-purpose RS-485 ports run Modbus RTU (or BACnet MS/TP) networks like a JACE's, while its dedicated eBMS/IO port carries expansion I/O modules, so adding I/O does not consume a third-party Modbus port. IONA also supports Modbus TCP over its two Ethernet ports. See IONA I/O and expansion.
Resilience: do not put all your eggs on one trunk
A Modbus RTU trunk is a single shared bus. A shorted cable, a failed transceiver or a device stuck transmitting can take down every device on it. Design so a single trunk failure has limited impact:
- keep critical plant (boilers, chillers, pressurisation) on different trunks from large populations of meters or terminal devices
- split large buildings into trunks by floor, riser or plant room, so a fault is contained to one area
- make sure plant controllers run safely on their own if communication is lost, with fail-safe settings for any outputs commanded over Modbus
- set alarms on device and network status so a trunk failure is noticed immediately, not at the next monthly report
Configuring a Modbus RTU network in Niagara
For each RS-485 port:
- Add a Modbus async network from the Modbus palette to the station's Drivers.
- In its serial port configuration, select the physical port (the port names correspond to the controller's onboard ports and option module ports), and set baud rate, data bits, parity and stop bits to match the devices.
- Set the response timeout and retries appropriate to the devices.
- Add devices with their unit IDs.
- Add points and assign tuning policies.
- Check the poll scheduler statistics under normal load.
For Modbus TCP, add a Modbus TCP network, then add devices with their IP address, port (normally 502) and unit ID.
Troubleshooting multi-network systems
- One network slow, others fine: check that network's poll scheduler statistics and devices for timeouts.
- Devices on a gateway time out intermittently: the gateway's response timeout is shorter than the serial devices need; increase the timeout on the Niagara network or gateway.
- TCP devices refuse connections: too many clients connecting to a device that allows only a few; make sure only one system polls it, or use a gateway.
- Wrong port assigned: check which physical port each network is bound to, especially after adding option modules.
Frequently asked questions
How many Modbus RTU networks can a JACE 9000 run?
One per RS-485 port: two onboard, plus the ports on up to two RS-485 option modules.
Can two Modbus networks share one RS-485 port?
No. An RS-485 port can host only one Modbus RTU network (one master), and cannot share with BACnet MS/TP.
How many Modbus TCP devices can a JACE handle?
There is no fixed per-network limit; the practical limits are licence capacity, controller resources, and how fast each device needs polling.
What is a Modbus TCP gateway network in Niagara?
A network that reaches Modbus RTU devices behind a TCP to RTU gateway, addressing each device by the gateway's IP and its unit ID.
Should I put all meters on one trunk?
Not if there are many. Split them across ports or gateways so polling stays responsive, and keep plant control devices on separate trunks from meters.
Can a JACE be a Modbus slave as well as a master?
Yes. Niagara can act as a Modbus server (slave) so other systems can read data from the station, for example over Modbus TCP. On a JACE 9000, an inbound Modbus TCP server port may need a manual firewall entry.
Can I use the same RS-485 port for Modbus and BACnet at different times?
No. A port runs one network continuously. Reassigning a port means moving every device on that trunk, so plan port allocation at design stage.
Is Modbus TCP faster than Modbus RTU?
Usually, because Ethernet is much faster than RS-485 and devices are polled independently. The devices' own response times then become the limit.
Do I need separate licences for each Modbus network?
The Modbus drivers are part of the controller's licence features; capacity is governed by Global Capacity points and devices across all networks. Check your licence includes the drivers you need.
Does IONA use a Modbus port for its I/O expansion?
No. IONA has a dedicated eBMS/IO RS-485 port for expansion I/O, leaving its two general-purpose RS-485 ports free.
Should meters and plant controllers share a Modbus trunk?
Preferably not on large systems. Meters are numerous and read slowly; plant controllers need responsive control. Separate trunks keep each group performing well and limit the impact of a fault.
How do I know which serial port name maps to which physical port?
Check the controller's documentation for port naming, and confirm by connecting a single known device to a port and testing communication before connecting the rest of the trunk.
Can a JACE use a USB-to-RS-485 adapter for extra Modbus networks?
Use the controller's supported option modules for additional ports. Supported hardware is designed and tested for the controller; improvised adapters are not.
Need help from a Niagara engineer?
If this guide has not solved the problem, contact the Tyrrell Products technical team on 01942 732819 or [email protected]. Tell us the controller or host type, its Niagara version and what you have already tried, and we will help you get the system running.