Description
Key Technical Specifications
| Parameter | Specification |
|---|---|
| Communication network | Modicon S908 Remote I/O |
| Communication rate | 1.544 Mbit/s |
| Communication method | Biphase-level modulation |
| Message format | HDLC |
| Coaxial connections | 2 F-type coaxial connectors |
| Cable arrangement | Dual-cable / redundant path |
| Power supply | +5 VDC |
| Current consumption | 1,500 mA at +5 VDC |
| Operating temperature | 0 to +60°C |
| Relative humidity | 0–95%, non-condensing |
| Shock resistance | 10 G, 11 ms |
| Mounting | One option slot in H8xx-209 housing |
| Weight | Approximately 1.2 kg |
Product Introduction
The Modicon AS-S908-120RP is identified by its base model as a dual-cable S908 Remote I/O processor for legacy Modicon 984 PLC systems. It provides the communication interface between the supervisory controller and remote I/O drops, allowing field signals to be exchanged with the central control system over the S908 network.
The dual-cable arrangement is intended for systems requiring an alternate communication path. The processor uses two coaxial connections and operates at 1.544 Mbit/s using biphase-level modulation and HDLC message formatting. It is a specialized legacy communication component; it should not be confused with Ethernet-based remote I/O equipment or newer Modicon communication modules.
AS-S908-120RP
Application Scenarios & Field Pitfalls
Typical applications
- Modicon 984-685 and 984-785 PLC remote I/O networks.
- Distributed 800-Series I/O installations.
- Dual-cable S908 communication architectures.
- Legacy process control systems requiring maintenance or replacement of an existing remote I/O processor.
Important engineering checks
- Controller compatibility: The technical manual identifies AS-S908-120 as a slot-mounted processor for compatible 984-685/785 controllers. Do not assume compatibility with every 984 model.
- Executive cartridge: The documented configuration requires the AS-E908-131 Executive Cartridge. Confirm the installed cartridge and supported drop configuration.
- Cable topology: Both coaxial paths, connectors, cable condition, and network termination must be checked. A dual-cable processor does not by itself guarantee a functioning redundant network.
- Power supply: Verify the +5 VDC backplane supply can support the processor’s 1.5 A demand.
- Network configuration: Confirm the number of remote drops and the installed I/O architecture before replacing the processor.
Related Products
| Model | Function | Relationship |
|---|---|---|
| AS–110 | Single-cable remote I/O processor | Single-cable counterpart |
| AS-E908-131 | Executive Cartridge | Required configuration component |
| AS-E908-016 | Executive Cartridge | Alternative drop-capacity configuration |
| AS-S911-800 | 984 Hot Standby module | Separate standby function, not a replacement |
The AS–110 is not a direct substitute when the installed system requires dual-cable communication.
Procurement and Quality Control
The AS–120 is a discontinued legacy component, so procurement should focus on exact model identification and functional condition rather than appearance alone.
Before accepting an AS–120RP, request:
- Front and rear photographs showing the complete part number.
- Confirmation of whether
RPis printed on the original OEM label. - Connector inspection for damage, corrosion, or looseness.
- Verification of the +5 VDC power requirement.
- A functional communication test with a compatible 984/ system, if available.
- Test documentation identifying the test setup and actual results.
Replacement recommendation: Match the original AS–120 hardware and confirm the installed executive cartridge and dual-cable network arrangement before installation. A generic power-on test is not sufficient to establish remote I/O communication reliability.
