24 Heat Zones, 24 Limits, Two RMA PLUS Gateways: Building a Watlow EZ-ZONE RM System

Applications

Sep 28

Some thermal systems need a lot of safety points. Others need a clean handoff to a PLC. This one needed both: 24 heated zones under PID control, a dedicated over-temperature limit on every one of those zones, SCR power switching for every zone, and all of it, power controllers included, visible to the plant PLC over Ethernet. Here's how we configured it with the Watlow EZ-ZONE RM family and ASPYRE AT power controllers, and what we learned about letting module sizes drive the bill of materials.

The problem

The requirements came down to four jobs:

Control: 24 temperature control loops.
Protection: 24 limit inputs, one per zone, independent of the control loops. When any zone goes over temperature, its limit trips regardless of what the control loop is doing.
Power: one power controller per zone, switching single-phase 480 VAC heater loads.
Communication: every control, limit and power device reachable from the customer's PLC over EtherNet/IP.

You could build that from 24 single-loop controllers, 24 standalone limit controllers and a pile of wiring. That means a big panel, a lot of terminations and a lot of places for mistakes. A modular, DIN-rail system that shares a bus is a much tidier answer.

Why the EZ-ZONE RM family

Watlow's EZ-ZONE RM is a modular, DIN-rail controller family designed to mix and match. Per Watlow, one system scales from one to 152 control loops and limits. The pieces we used:

RMH high-density control module. Watlow offers it with 4, 8, 12 or 16 PID loops; the loop count is set by the part number.
RML high-density limit module. An FM-approved over/under temperature limit module with 4, 8 or 12 limit loops per module, again set by the part number. We quoted universal inputs (thermocouple, RTD or process signal), so the customer isn't locked into one sensor type.
RMA PLUS access module. The gateway to the outside world. Modbus TCP is standard on every RMA PLUS; EtherNet/IP is only available on the EtherNet/IP model (RMAP-X3XX-XXXX), so that digit matters when you order. Configured as an EtherNet/IP server, it presents the devices behind it to the PLC. This system uses two, and both must be the EtherNet/IP model.
ASPYRE AT SCR power controllers. One per control loop, on a DIN-rail heatsink. Watlow lists ASPYRE AT as interoperable with the RMA PLUS, so all 24 sit on the communications bus and their data reaches the PLC through the gateways.

Watlow configures the whole ecosystem with its COMPOSER software. The limit modules, control loops and power controllers are set up as one system instead of as separate boxes.

Let the module sizes drive the count

The most useful part of this job was, again, simple arithmetic.

Control loops: 24 zones on RMH modules. Two 12-loop RMH modules cover it exactly, with no spare loops. If the customer wants room to grow, a 16-loop plus an 8-loop RMH also gives 24, and two 16-loop modules gives 32 (eight spare). We quoted two 12-loop modules and listed the 16 + 16 option.

Limits: an RML tops out at 12 limit loops, so 24 limits means two 12-limit RML modules, fully used. Each zone's limit sensor lands on its own RML channel, separate from the control sensor.

Relay outputs: every RML has an output slot (Slot E) that can't be left empty. Watlow offers two choices there: four 5 A Form A relays, or one digital input plus two relays (one Form A, one Form C). With two RML modules, that gives 8 relay outputs (both four-relay), 6 (one of each) or 4 relays plus 2 digital inputs (both two-relay). The right choice depends on how the limit trips are wired into the heater contactors and how many remote reset inputs the customer wants, so we confirm it per job.

Power: 24 zones, 24 ASPYRE AT units. One power controller per loop keeps troubleshooting simple, because a zone problem stays in that zone. The ASPYRE AT current rating is picked from each zone's heater load.

Gateways: in Watlow and Entherm application experience, one RMA PLUS supports up to 15 devices. With every power controller on the bus, the device count is 2 RMH + 2 RML + 24 ASPYRE AT = 28 devices. 28 / 15 rounds up to two RMA PLUS gateways. We split them evenly:

RMA PLUS #1: 2 RMH + 2 RML + 10 ASPYRE AT = 14 devices
RMA PLUS #2: 14 ASPYRE AT = 14 devices

That leaves one open position on each gateway for a future device.

Two connections to the PLC

Each RMA PLUS is its own EtherNet/IP connection to the PLC, with its own IP address and its own set of assemblies in the PLC program. Both gateways need the EtherNet/IP model (RMAP-X3XX-XXXX). Modbus TCP alone won't do it.

With the ASPYREs on the bus, the PLC can see per-zone power data alongside the control loops and limits. How each ASPYRE AT gets its power command depends on the options ordered: a direct control signal from the zone's RMH output, or a command over communications. That's set by the RMH output and ASPYRE AT input options, so confirm the signal path with Entherm when you configure the system.

The resulting bill of materials: two RMH, two RML, two RMA PLUS (EtherNet/IP model) and 24 ASPYRE AT. Four RM modules handle every control loop and every limit, a job that would otherwise take 48 separate controllers, and two gateways put all 28 devices on the network.

What's not in the box

A module list isn't a finished control system. We called out what the customer or panel builder still has to plan for:

A 24 VDC power supply for the RM modules.
Fusing and fuse holders for 24 SCR branch circuits, sized for the panel's short-circuit rating.
Control wiring from RMH outputs to ASPYRE AT inputs, if the chosen options use a direct control signal.
An Ethernet switch, or two PLC ports, for the two gateway connections.
Heater contactors or another means for the limit relays to remove power, since 24 zones need a clear plan for what each limit trip shuts off.
DIN rail, the enclosure and panel wiring. Twenty-four SCRs on heatsinks take real panel space and heat removal.
An HMI or SCADA package if operators need more than the PLC screens. SpecView is one option Entherm supports.

Calling these out early avoids the most common surprise in a controls project: a quote that looked complete but left out the pieces between the components.

Why this approach works

Safety stays independent. Every zone gets its own limit in dedicated limit modules, separate from the PID loops.
Full visibility. The PLC sees every control loop, limit and power controller through two EtherNet/IP gateways, not a daisy chain of serial devices.
Room to grow. Each gateway carries 14 of the 15 devices it supports, and larger RMH options add spare loops without adding modules.
Less wiring. Modules share the bus and mount side by side on DIN rail.

Request a quote

Planning a multi-zone heating system with limits, power control and a PLC connection? Tell us your zone count, limit count, heater voltage and current, and which network your PLC speaks. We'll configure the Watlow system and give you a clear list of what's included and what isn't.

Email sales@entherm.com or call 1-877-368-4376.

Sources

David Schwanke