Modicon AS-S908-120RP | Remote I/O (RIO) Drop Adapter | 155m Range

Description

Key Technical Specifications

Product Introduction

If you are maintaining a legacy Modicon system—maybe a 984 PLC or an early A120 setup—you know the S908 RIO bus is the backbone that refuses to die. The AS-S908-120RP is the bridge. It sits in your local rack and talks to your remote drop adapters (like the AM-S908-000) over a simple coaxial cable.I’ve seen these networks run for 20 years in steel mills. The beauty of this module is its simplicity. It doesn’t need an IP address, no Ethernet switch, and no complex routing tables. It’s just a raw, deterministic bus. The “120” in the model number generally refers to the specific generation of the A120 platform interface. It handles the heavy lifting of serialization, turning your parallel backplane data into a serial stream that shoots down the coax cable. It’s durable, but the connectors are the weak point. If the BNC connector gets wiggled loose, your remote I/O vanishes instantly.

Quality SOP & Tech Pitfalls (The Reality Check)

The Lab Report (SOP)
We don’t just ship these because they light up.

1. Visual Inspection: We check the BNC connector. On these old modules, the center pin often gets bent or the connector body gets cracked. We also check the backplane pins for corrosion.
2. Backplane Test: We install it in an A120 chassis. We verify it draws the correct current (around 1A) and doesn’t short the 5V rail.
3. Loopback Test: We connect the RIO port to a master simulator. We verify it can handshake and pass data blocks. We check the “Comm” LED behavior against the datasheet timing.
4. Cleaning: These often come from dirty environments. We clean the board with contact cleaner to remove conductive dust.

The Engineer’s Warning (Pitfalls)
The “RP” suffix and the termination switch. This module has a termination resistor (usually 150 Ohms) that must be enabled only if this module is at the very end of the RIO daisy chain. I’ve seen technicians leave the termination switch “ON” in the middle of a chain. This kills the signal reflection and causes intermittent communication faults that drive you crazy. Check the switch setting. Also, S908 RIO is notorious for ground loops. If your local rack and remote rack have different ground potentials, you will fry the transceiver. Always use an isolated repeater (fiber optic is best) if the distance is over 100m or if the grounds are suspect.

Installation & Configuration Guide

1. Pre-Installation (Safety First)

• Power Down: Turn off the A120 rack power. This is not hot-swappable in the modern sense; pulling it live can crash the PLC scan.
• Check Cabling: Inspect your RG-6/U coaxial cable. Ensure the BNC connectors are crimped correctly. A loose shield is a noise magnet.

2. Removal

• Unscrew the BNC connector (if connected).
• Release the locking tab on the module faceplate.
• Slide the module out of the chassis.

3. Installation

• seating: Slide the AS-S908-120RP into the slot. Ensure it seats firmly.
• Configuration: Check the physical switch on the module (if present) or the software configuration in Modsoft/Concept. Ensure the “Drop Number” matches what the Master (AM-S908-000) expects.
• Termination: Set the termination switch. ON only if this is the last device on the cable. OFF if the cable continues to another drop.

4. Power-On & Testing

• Apply power.
• Watch the LEDs. PWR should be solid. RIO should flash or turn solid green once the Master polls it.
• If you see a red FAULT LED, check your Drop ID settings and termination.

Compatible Replacement Models

Frequently Asked Questions (FAQ)

1. What does the “RP” stand for?
In Modicon part numbers, it often denotes a specific revision or packaging, but functionally, this is a standard S908 RIO drop adapter. Don’t confuse it with “Repeater”—this module is an endpoint, not a signal booster.2. Can I use fiber optic cable with this?
No. The AS-S908-120RP has a BNC connector for copper coax. If you need to go fiber (for distance or isolation), you need to buy a separate media converter (like the Ci-rf series) and connect it to this module’s BNC port.3. Why is my RIO communication intermittent?
90% of the time, it’s the BNC connector. The center pin gets loose, or the cable shield isn’t making contact. Wiggle the cable; if the fault light flickers, re-crimp your connectors. The other 10% is ground loops—check your earth grounds.4. How far can I run this without a repeater?
At 1.544 Mbps, you are limited to about 155 meters (500 feet). If you drop the speed (which you can do in the Master configuration), you can go further, but 1.544 Mbps is the standard “fast” rate.5. Does this module need an IP address?
No. S908 RIO is a pre-Ethernet protocol. It uses “Drop Numbers” (0-76) to identify devices. You configure the Drop Number in your PLC logic (Modsoft/Concept) to match the physical setting or logic assignment.6. Is this module isolated?
It has basic isolation, but for long runs in noisy plants (like near VFDs), the internal isolation might not be enough. If you see blown transistors on the RIO port, install an external isolation repeater.