StarTech PEX4S953LP 4-Port PCIe RS-232 Serial Adapter Card

Overview

The StarTech PEX4S953LP is a low-profile PCI Express serial adapter that adds four RS-232 DB-9 ports to any PCIe 1.1-equipped server or workstation. If you're integrating legacy serial devices — access control panels, industrial PLCs, serial-based PTZ camera controllers, or Wiegand bridges — into a modern rack server that has no native COM ports, this card closes that gap without requiring a full-height slot. At 3.1 x 2.4 inches, it fits both standard and small-form-factor chassis, which makes it practical for compact edge deployments where rack space is at a premium.

Key Features

• Four DB-9 RS-232 Ports on a Single PCIe Slot: Consolidating four serial channels onto one slot means you're not sacrificing multiple expansion slots in a tight server. Each port is independently configurable for baud rate, data bits, and flow control — critical when you're running mixed serial devices with different communication profiles on the same host.
• 921.6 Kbps Maximum Baud Rate: Most legacy RS-232 gear tops out at 115.2 Kbps. The PEX4S953LP's ceiling of 921.6 Kbps gives you headroom for high-throughput serial devices — barcode verifiers, some industrial automation controllers, or serial-based aggregators — without being a bottleneck on the host side.
• 256-Byte FIFO Buffer per Channel: The onboard 256-byte FIFO absorbs burst data from each port without dropping characters under CPU load. In practice, this matters most in multi-threaded environments where OS interrupt latency isn't perfectly consistent — the buffer keeps the data intact until the driver polls it.
• ASIX AX99100A Chipset: The AX99100A is a well-documented chipset with broad OS driver support across Windows and Linux distributions. For integrators deploying this in Linux-based VMS servers or custom embedded environments, driver availability is a real operational concern — not an afterthought.
• 5V/12V Power Output on Pin 9: This is a detail that matters in specific installations. Certain serial peripherals — particularly older PTZ receivers and some industrial sensors — draw low-voltage power directly from the serial connector's pin 9 rather than requiring a separate power supply. Having selectable 5V or 12V on pin 9 eliminates the need for an auxiliary power tap at the device end.
• Flexible Data Framing (5–9 Data Bits, Multiple Stop Bits): Supporting 5, 6, 7, 8, and 9 data bit configurations alongside 1, 1.5, and 2 stop bits means the card isn't locked into 8N1 only. Older Pelco-D and similar protocol implementations occasionally use non-standard framing — this card handles those edge cases without a protocol converter in the middle.
• Hardware and Software Flow Control: Supporting None, XON/XOFF, and RTS/CTS means you can match the flow control method to whatever the downstream device requires. RTS/CTS is mandatory for some industrial serial protocols; XON/XOFF is common in legacy terminal-style devices. Having both covered on the same card avoids device-specific workarounds.
• Low-Profile Form Factor (3.1 x 0.6 x 2.4 in): At under 2.5 inches tall, this card installs directly in small-form-factor servers and 1U rack units with low-profile PCIe slots. No bracket swap needed if the chassis already ships with a low-profile bracket — check whether the card ships with both brackets before ordering.
• PCI Express 1.1 Compliance: PCIe 1.1 is backward and forward compatible with PCIe 2.0 and PCIe 3.0/4.0 slots. You can drop this into virtually any server manufactured in the last 15 years without a compatibility concern on the host interface side.
• Operating Temperature Range -10°C to 80°C: Rated to 80°C on the high end, the card tolerates the interior of a 1U rack unit in a poorly cooled telecom closet without thermal throttling or failure — useful context for edge deployments without dedicated HVAC.
• Lifetime Warranty: The card carries a lifetime manufacturer warranty, which reduces the total cost of ownership argument compared to commodity serial adapters that ship with one- or two-year coverage. For a card sitting in a production server with a five-year or longer depreciation cycle, this matters.

Integration and Compatibility

The PEX4S953LP targets environments where a modern PCIe host — server, workstation, or industrial PC — needs to communicate with RS-232 serial devices that lack USB or Ethernet alternatives. Common integration points include access control head-end panels with RS-232 configuration ports, serial PTZ camera controllers operating on Pelco-D/Pelco-P protocols, industrial automation equipment with serial command interfaces, and legacy building management system (BMS) nodes. The ASIX AX99100A chipset has published driver support across Windows Server editions and mainstream Linux kernel versions, which covers the majority of VMS server and industrial PC operating environments. For deployments running 32-bit or embedded OS builds, verify driver availability for your specific OS version before specifying this card. The PCIe 1.1 interface is slot-compatible with PCIe x1, x4, x8, and x16 physical slots — standard across virtually all server and workstation motherboards.

Frequently Asked Questions

Q: Does the PEX4S953LP require a full-height PCIe slot?

A: No. The PEX4S953LP is a low-profile card measuring 3.1 x 2.4 inches, designed to fit in low-profile PCIe slots found in 1U rack servers and small-form-factor workstations. Verify whether a low-profile bracket is included before installation in a standard full-height chassis.

Q: What is the maximum baud rate supported per port?

A: Each port supports up to 921.6 Kbps — well above the 115.2 Kbps ceiling of most legacy RS-232 devices, giving you headroom for higher-throughput serial peripherals.

Q: What chipset does the PEX4S953LP use, and does it have Linux driver support?

A: The card uses the ASIX AX99100A chipset, which has documented driver support for both Windows and Linux environments. Confirm driver availability for your specific kernel or OS version before deploying in embedded or non-standard Linux builds.

Q: Can the PEX4S953LP supply power to serial devices through the connector?

A: Yes. Pin 9 on each DB-9 connector carries selectable 5V or 12V output, which allows certain serial peripherals that draw power directly from the RS-232 connector to operate without a separate power supply.

Q: What is the warranty on the PEX4S953LP?

A: The PEX4S953LP carries a lifetime manufacturer warranty.

Q: Is the PEX4S953LP compatible with PCIe 2.0 and PCIe 3.0 motherboards?

A: Yes. PCIe 1.1 cards are backward and forward compatible with PCIe 2.0, 3.0, and 4.0 slots due to the PCIe standard's lane compatibility design. The card will install and operate in any standard PCIe x1 or larger slot.

James Everett

The spec I keep coming back to on the PEX4S953LP is the 256-byte FIFO buffer per channel combined with the 921.6 Kbps baud ceiling. In practice, most integrators never push past 115.2 Kbps on RS-232, but the buffer depth is what actually saves you in a busy server environment where interrupt service isn't instantaneous — you stop losing bytes mid-frame on Pelco-D strings or access control config traffic.

Technical Highlights:

• ASIX AX99100A Chipset: Widely supported across Windows Server and Linux kernels — driver sourcing is rarely a problem, which matters when you're deploying into a custom-built VMS server or an embedded Linux head-end.
• Pin 9 Power Output (5V/12V): Eliminates the need for auxiliary power taps at legacy serial PTZ receivers and some older access control nodes that pull low-voltage power directly from the DB-9 connector — a genuine wiring simplification in tight enclosures.
• Low-Profile PCIe 1.1 Form Factor (3.1 x 2.4 in): Fits directly in 1U rack servers without a bracket swap, and the PCIe 1.1 interface slots into PCIe 2.0, 3.0, and 4.0 hosts without any compatibility friction.

Deployment Considerations:

• Confirm your chassis ships with a low-profile bracket if you're installing in a 1U server — some chassis only include a standard full-height bracket and the card height won't clear the rear panel without the correct bracket installed.
• For embedded Linux or non-standard OS builds, validate AX99100A driver availability for your specific kernel version before specifying this card — the chipset is well-supported on mainstream distributions but edge-case OS builds occasionally require manual driver compilation.

The PEX4S953LP is the right call for a physical security head-end server that needs to talk RS-232 to legacy PTZ controllers, serial-based access control panels, or building automation nodes — particularly in a 1U rack deployment where low-profile slot constraints rule out full-height alternatives.