Two communication ports support RS-422 point-to-point connections and RS-485 multipoint connections.
Model: ALR121-S50 Type: Standard / No Explosion Protection / Basic Type
PRODUCT DETAIL
Communication Architecture Built Around Two Serial Paths
01 / MODULE ROLE
ALR121-S50 for RS-422 / RS-485 Serial Communication
The Yokogawa ALR121 is a two-port serial communication module designed for use with Yokogawa safety control stations. It provides RS-422/RS-485 communication using a 4-wire system and supports point-to-point communication through RS-422 or multipoint communication through RS-485.
The module operates with half-duplex communication and start-stop synchronization. Its serial communication capability can be used for Modbus communication and subsystem communication within supported Yokogawa control-system configurations.
Two serial communication ports are provided on the ALR121 module.
02
Long Serial Reach
The specified maximum transmission distance is 1200 m total extended length.
03
Modbus Capability
The supported configuration includes Modbus RTU communication for the applicable communication functions.
04
Redundant Paths
A dual-redundant communication configuration can be established using two communication modules.
TECHNICAL PROFILE
ALR121-S50 Specifications
Model
ALR121-S50
Module Type
Serial Communication Module, RS-422/RS-485, 2-port
Interface
RS-422 / RS-485, 4-wire system
Connection Method
Point-to-point for RS-422; multipoint for RS-485
Communication
Half-duplex
Synchronization
Start-stop synchronization
Transmission Speed
1200 / 2400 / 4800 / 9600 / 19200 / 38400 bps
Transmission Code
Binary
Character Length
8 bits; fixed to 8 bits for the Modbus slave and subsystem communication functions
Stop Bit
1 / 2 bits; fixed to 1 bit for the subsystem communication function
Parity
None / Even / Odd
Transmission Distance
Maximum 1200 m total extended length
I/O Wiring
Cable with 3-pair shield; AKB161 or AKB162
Wiring Connection
M4 screw terminal block, 5 poles × 2 ports
Current Consumption
0.5 A
Weight
Approx. 0.34 kg
COMMUNICATION PATH
From Serial Port to Control Logic
ALR121 can operate as part of the communication path between a Yokogawa safety control station and an external Modbus device or subsystem.
Modbus Slave Communication
In the applicable ProSafe-RS configuration, the SCS can act as a Modbus slave and communicate with a Modbus master in another system through the serial communication module.
Subsystem Communication
The SCS can also use the serial communication module to exchange Modbus RTU data with a subsystem configured as a Modbus slave.
01
Modbus Master
External System
→
ALR121
2-Port Serial Module
→
03
SCS
Safety Control Station
RS-422RS-485Modbus RTUHalf-Duplex
02
REDUNDANT PATH
Dual-Redundant Communication Configuration
A dual-redundant configuration can be established using two ALR121 communication modules with independent transmission paths from the Modbus master. In the redundant arrangement, one module operates as the control side while the other remains on standby.
When a communication fault is detected, the control can switch to the standby module so communication can continue through the alternate path.
Control
ALR121 Module A
↕
Standby
ALR121 Module B
SAME SERIES
ALR121 Series Position
ALR121
Two-port serial communication module supporting RS-422/RS-485 communication.
The companion serial communication module uses RS-232C with two ports, providing a different physical interface from ALR121.
RS-232C · 2 Ports · Point-to-point
Selection focus: ALR121 is the relevant series choice when the serial interface requirement is RS-422/RS-485 rather than RS-232C.
PURCHASE DETAILS
Prepared for Industrial Replacement & Procurement
For quotation matching, please provide the exact part number together with the quantity, destination, required date, and the machine or system where the module will be installed.
Can ALR121-S50 be used when the existing serial network requires RS-485 multipoint communication?
Yes. ALR121 supports RS-485 multipoint communication. The module uses a 4-wire RS-422/RS-485 interface, so the existing wiring and communication configuration should be checked against the actual system arrangement before replacement.
Does ALR121-S50 support a long-distance serial link in an existing plant installation?
The specified maximum transmission distance for ALR121 is 1200 m total extended length. Actual replacement planning should still account for the installed cable arrangement and the communication configuration.
Is ALR121-S50 suitable for a Modbus RTU connection between a Yokogawa SCS and another system?
The ALR121 communication architecture supports Modbus RTU. In the applicable ProSafe-RS configuration, the SCS can communicate with an external Modbus master, while the subsystem communication function allows the SCS to communicate with a Modbus slave subsystem.
What should be checked before replacing an ALR121-S50 in a redundant communication arrangement?
Confirm that the existing arrangement uses the ALR121 communication architecture and that the replacement module matches the required RS-422/RS-485 connection, communication function and system configuration. A dual-redundant arrangement uses two communication modules with independent transmission paths.
Does the ALR121-S50 suffix matter when replacing an older ALR121 configuration?
Yes. ALR121-S50 is a specific suffix configuration: “-S” denotes the standard type, “5” denotes the version without explosion protection, and “0” denotes the basic type. When replacing an installed unit, the complete part number should be matched rather than selecting only by the ALR121 family name.
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