StarTech PEX4S232485 4-Port PCIe Serial Expansion Card (RS-232/422/485)

Overview

The StarTech PEX4S232485 is a PCI Express x1 serial expansion card that adds two RS-232 ports and two RS-422/RS-485 ports to any desktop PC or server with an available PCIe slot. If you're running industrial control systems, serial-attached POS terminals, barcode scanners, PLCs, or legacy serial devices alongside a modern PC platform, this card eliminates the need for external USB-to-serial adapters that introduce latency and driver instability. All four ports are exposed as standard DB-9 connectors, keeping cabling familiar and straightforward. The card ships in a standard profile form factor, fitting full-size desktop and rackmount chassis without modification.

For integrators and IT architects connecting serial expansion cards to mixed industrial device pools, the PEX4S232485 is a practical, low-overhead solution that keeps serial traffic on a dedicated PCIe bus rather than routed through USB arbitration.

Key Features

• Dual Protocol Support (RS-232 and RS-422/485): Two ports operate in RS-232 mode for point-to-point connections to devices like modems, POS terminals, and serial consoles, while the other two ports support RS-422 and RS-485 — the protocols used in industrial automation, security panel buses, and long-distance multi-drop serial networks. One card covers both environments, reducing the number of expansion slots consumed.
• PCIe x1 Interface: Connecting over PCI Express rather than USB means no USB host controller congestion, no COM port enumeration conflicts, and no dropped characters under high CPU load. The PCIe bus provides dedicated bandwidth and direct interrupt handling, which matters in time-sensitive industrial control loops.
• 460.8 Kbps Maximum Baud Rate: Supports baud rates up to 460.8 Kbps — fast enough for most industrial serial protocols and high-speed serial device communication. If you're running Modbus RTU, DMX512, or access control panel buses, you'll have headroom above the typical 9600–115200 bps requirements of those protocols.
• 256-Byte FIFO Buffers: Each port includes a 256-byte FIFO buffer, which cushions burst serial data and reduces the risk of character overruns when the OS is momentarily busy. Compared to older 16-byte FIFO UARTs, this significantly reduces dropped-byte errors in high-throughput or multi-port concurrent scenarios.
• Flexible Flow Control Options: Supports None, XON/XOFF software flow control, and RTS/CTS hardware flow control. This covers the full range of handshaking requirements across legacy and modern serial devices — you won't need to compromise on flow control configuration for a specific device type.
• Full Parity and Stop Bit Configuration: Parity options include None, Odd, Even, Space, and Mark. Stop bits support 1, 1.5, and 2 configurations. That level of configurability means compatibility with virtually any serial device specification you'll encounter in an older industrial or building automation install.
• Standard Profile Form Factor: At 5.0 in × 4.1 in × 0.6 in and 2.4 oz, the card fits standard desktop towers and 4U-and-above rackmount chassis. It won't fit low-profile (half-height) cases — verify chassis clearance before ordering.
• 12V DC Operation: Powered directly from the PCIe slot at 12V DC, with no external power connector required. One less cable to manage during install.
• Windows and Windows Server OS Support: Driver support covers Windows desktop and Windows Server editions, making it deployable in both workstation and server-rack environments running Windows-based SCADA, VMS, or automation software.
• Lifetime Warranty: StarTech backs the PEX4S232485 with a lifetime warranty — meaningful for a card that may be installed in a system expected to run for a decade or more in an industrial or building automation environment.

Integration and Compatibility

The PEX4S232485 installs into any available PCIe x1, x4, x8, or x16 slot — PCIe slots are backward and forward compatible at the physical level for x1 cards. Once drivers are installed, each port appears as a standard COM port in Windows Device Manager, fully compatible with any application that communicates via the Windows serial port API. This includes SCADA platforms, VMS servers with serial-connected cameras or panel integrations, serial-to-IP gateways, and building automation controllers.

RS-485 multi-drop wiring supports up to 32 devices per port in standard configurations, making this card suitable for running Modbus RTU device buses or RS-485-based access control panel networks directly from a host PC. For applications requiring RS-422 full-duplex point-to-point serial at distances up to 1200 meters, the RS-422 ports handle that without external line drivers.

This card pairs naturally with industrial serial adapters and serial device servers when extending coverage beyond the host PC. If you're building out a serial infrastructure for a PCIe expansion stack, review your chassis slot availability and OS driver requirements before committing.

Buyers evaluating serial connectivity infrastructure should also reference a serial port selection guide to confirm protocol requirements before specifying RS-232 versus RS-422/485 port allocation.

Frequently Asked Questions

Q: What is the difference between the RS-232 and RS-422/485 ports on the PEX4S232485?

A: The two RS-232 ports are designed for point-to-point serial connections at shorter cable runs — typical for connecting a single device like a modem, POS terminal, or serial console. The two RS-422/485 ports support longer cable runs (up to 1200m for RS-422) and RS-485 multi-drop bus configurations where multiple devices share a single serial line, common in industrial automation and building control systems.

Q: Does the PEX4S232485 fit in a low-profile (half-height) PC case?

A: No. The PEX4S232485 ships in standard profile form factor only (5.0 in × 4.1 in). It requires a full-height PCIe slot. If your chassis is low-profile, this card will not physically fit without a bracket swap — and no low-profile bracket is confirmed in the evidence for this model.

Q: What operating systems are supported by the PEX4S232485?

A: The PEX4S232485 supports Windows and Windows Server editions. Driver support for Linux or macOS is not confirmed in available product evidence — verify with the manufacturer if those platforms are required.

Q: What is the maximum baud rate supported by the PEX4S232485?

A: The card supports a maximum baud rate of 460.8 Kbps across all four ports. This exceeds the 115200 bps standard maximum of most legacy serial devices and provides headroom for higher-speed serial protocols where supported by the connected device.

Q: What PCIe slot does the PEX4S232485 require?

A: The card uses a PCIe x1 connector. PCIe x1 cards are electrically and physically compatible with x1, x4, x8, and x16 slots, so slot availability is rarely a constraint in a modern desktop or server chassis.

Q: What warranty does the PEX4S232485 carry?

A: StarTech provides a lifetime warranty on the PEX4S232485.

Karl Wilson

The PEX4S232485 is the card I'd reach for when a site has a mix of legacy RS-232 devices — think serial-attached access control panels or older POS hardware — alongside RS-485 bus-connected field devices like Modbus RTU sensors or building automation controllers. The split of two RS-232 and two RS-422/485 ports is a deliberate design choice that reflects real-world mixed serial environments, and the 256-byte FIFO buffers on each port mean you're not fighting character overruns on a busy Windows SCADA host the way you would with a basic 16-byte UART implementation.

Technical Highlights:

• 460.8 Kbps Baud Rate Ceiling: Four times the 115.2 Kbps standard ceiling — practical headroom for protocols that push above the conventional maximum, without requiring a specialty high-speed serial card.
• 256-Byte FIFO per Port: At 16x the buffer depth of a standard UART, burst serial data from multiple concurrent ports is absorbed without dropped bytes — relevant when polling multiple Modbus devices in rapid succession.
• Lifetime Warranty: For infrastructure cards that get installed and forgotten for years, a lifetime warranty removes the cost-of-replacement equation from the maintenance budget entirely.

Deployment Considerations:

• Operating temperature range is listed as 0°C to 100°C — confirm your chassis internal ambient stays within that range in a sealed industrial enclosure or hot server room without direct airflow across the PCIe slots.
• Windows and Windows Server driver support only per the evidence — if your SCADA or automation platform runs Linux, verify driver availability with StarTech before committing to this model.

The PEX4S232485 is the right pick for a Windows-based building automation workstation or security management server that needs to communicate with both legacy RS-232 peripheral devices and an RS-485 device bus in the same chassis, without consuming more than one PCIe slot or adding USB adapter clutter to the equation.