SX90 Process Controller

SX90 process controller for panel-mounted PID control

The Spirax Sarco SX90 is a 1/8 DIN panel-mounted universal-input PID process controller for industrial temperature and pressure control. It accepts a wide range of process signals, applies configurable PID control, and provides outputs for modulating equipment, alarms, retransmission and logic functions. The controller is designed to work with pneumatic or electric control valves and with the electronic and electrical instruments that complete a process control loop. For a project searching for a Spirax Sarco SX90 process controller, the central selection task is to match the measured variable, control response, output arrangement and panel installation to the actual plant.

Flexible process signal path

The SX90 process controller has a universal process-variable input. Depending on the configured application, the input can accept a three-wire Pt100 RTD, thermocouples of types K, J, N, R, S, B, L, T and C, a custom thermocouple, a 4-20 mA or 0-20 mA linear signal, or a -10 to 80 mV signal. This lets one panel format serve temperature instruments and transmitter-based pressure or other process measurements without adding a separate signal converter for every input type. The mA input uses the supplied 2.49 ohm shunt resistor as shown in the wiring information, so the complete signal circuit must be checked before commissioning.

The controller can work with three local setpoints and a remote setpoint. A remote setpoint signal can be supplied as 0-10 V or 0-20 mA, allowing the operating target to be set from another control layer. The documented cascade function uses a remote setpoint to coordinate two controllers in a master/slave arrangement. This makes the SX90 useful where the local loop needs a stable target from a supervisory or upstream process loop, while keeping the controller's own input, output and alarm configuration close to the equipment being controlled.

Control functions that suit changing process demand

PID control gives the SX90 a way to respond to process error rather than simply switch at one threshold. Proportional, integral and derivative settings can be adjusted for the process dynamics, and automatic tuning can select control parameters from the observed process response. The controller also supports automatic, manual and off modes. Manual mode is useful during commissioning or controlled fault-finding, but the output level and the effect on the plant remain the responsibility of the authorised commissioning person.

Setpoint ramping provides separate rising and falling rate limits when the process should move between targets at a controlled rate. The SX90 can use an automatic or manual setpoint, a remote setpoint and event alarms for deviation, band or process-value conditions. Three logic inputs provide selectable functions such as automatic/manual selection or alarm acknowledgement. Text messages can be configured for operator prompts, and a force-output function can be activated through a logic input or parameter where the configured instrument settings permit it. These functions are useful for commissioning and operating visibility, but they do not replace an independent protective trip or a site safety system.

Outputs for valves, alarms and control-system integration

The output hardware is arranged for more than one type of final control action. The standard output set includes a normally open relay, configurable analogue outputs and an SX90-specific changeover relay. Outputs 5 and 6 are interlocked normally open relays for VMD valve-motor drive operation. Depending on the quick-code configuration, the available outputs can be assigned to VMD control, continuous current or voltage functions, alarm relays, logic or retransmission. The exact quick code must therefore be selected from the required valve, actuator and alarm arrangement rather than inferred from the product name alone.

OP2 and OP3 can provide 0-20 mA or 4-20 mA analogue signals. The relay contacts are rated for a maximum 2 A at 264 Vac resistive load, with the terminal and load restrictions in the wiring instructions applying to the installed circuit. A potentiometer input accepts a 100-10 kOhm feedback potentiometer and can be used where the valve position needs to be brought into the control arrangement. An isolated remote-setpoint input accepts either voltage or current. The built-in RS485 five-wire interface is compatible with EIA422 wiring and Modbus RTU slave operation, with the technical sheet documenting connection of up to 64 controllers to a management system.

Panel fit and operating boundary

The SX90 is intended for permanent indoor installation in an electrical panel. Its approximate envelope is 48 mm wide, 96 mm high and 101 mm deep; the technical drawing also identifies a 90 mm rear-body dimension. The recommended panel cutout is 45 mm by 92 mm with the stated manufacturing tolerances. The front panel is rated IP65 when inserted through the panel and is identified as NEMA 4X in the exact-model technical sheet. These front-panel ratings do not remove the need for a suitable enclosure, clean wiring practice and protection from condensation, conductive pollution, vibration and electrical interference.

The documented supply range is 85-265 Vac with maximum power consumption of 9 W. The stated operating environment is 0-55 deg C and 5-85% relative humidity. The SX90 is not suitable for explosive or corrosive atmospheres. The installation guide additionally treats it as an indoor panel instrument and identifies an upper altitude boundary of 2000 m for the general product-family application. Where a process can overheat after a control fault, fit an independent overtemperature protection device; the controller's alarm relays are not a substitute for protection under every failure condition.

Selection checks before ordering

Start by defining the process variable and its complete signal path: sensor or transmitter type, range, cable arrangement, shielding and any required shunt resistor. Then decide whether the final action is a VMD valve-motor drive, a current or voltage signal, a relay function or a combination of control and alarm outputs. Confirm whether the loop needs one of the three local setpoints, a remote setpoint, cascade operation, setpoint ramping or manual operation during commissioning. Check the selected quick code against these decisions because it defines the input, control/output type, alarm-relay function, OP4 function and user-message language.

Panel design should allow the 1/8 DIN cutout, the rear depth and access to the plug-in screw terminals. Separate mains and low-level signal wiring, ground cable screens at the documented point, and provide the required external disconnect and overcurrent protection. Confirm relay loads, inductive-load suppression, the transmitter supply requirement and the actual communication topology before wiring. When ordering the Spirax Sarco SX90 for a loop that includes a control valve, select the valve, actuator and controller as one coordinated package. The positioners, controllers and sensors range provides the wider instrumentation context, while this page remains focused on the SX90 model.

The SX90 is a compact controller with broad signal and output capability, but successful operation depends on the complete engineered loop. When a project still needs to define the wider control system around the controller, review the valve, actuator, instrumentation and protection requirements together. Verify the instrument code, plant limits, protection layers, panel conditions and commissioning responsibility before releasing an order. The technical and installation sections that follow preserve the model-specific values, terminal functions, safety requirements and operating checks needed for that review.