MS-DOS 6.22 Virtual Serial Port Routing Drivers

Routing serial printer traffic to a network printer in MS-DOS 6.22 requires specialized Terminate-and-Stay-Resident (TSR) drivers or device-specific redirectors, because native DOS networking tools only redirect parallel (LPT) ports. While the standard Microsoft Network Client 3.0 relies on REDIR.EXE to capture LPT output and send it across a local area network, routing a virtual COM port across a network connection requires vendor-specific virtual serial redirectors—most notably the Lantronix COM Port Redirector (CPR for DOS) or Moxa NPort DOS drivers—or an INT 14h/INT 17h redirection bridge.

Native MS-DOS Network Limitations

In MS-DOS 6.22, network file and print sharing is typically implemented using the Microsoft Network Client 3.0 or Novell NetWare. The native Microsoft redirector (REDIR.EXE or the full redirector PROTMAN.DOS stack invoked via NET.EXE) allows users to map parallel printer output to network shares using the command:

NET USE LPT1: \\SERVER\SHARE

However, Microsoft’s native redirector does not support redirecting serial communication ports (COM1–COM4) over SMB/NetBIOS. If legacy DOS software is hardcoded to output print jobs to a serial port rather than a parallel port, NET USE alone cannot fulfill the request.

Vendor-Specific Network Serial Port Drivers

To route a virtual serial port directly over Ethernet (TCP/IP or IPX) to a network print server or serial device server, specialized third-party drivers must be loaded into memory via CONFIG.SYS or AUTOEXEC.BAT.

  1. Lantronix COM Port Redirector (CPR for DOS): Lantronix developed a dedicated MS-DOS TSR redirector for their network print and device servers (such as the EPS and ETS series). The driver operates over a packet driver or network interface card (NIC) shim, intercepting standard BIOS serial interrupts (INT 14h) or virtualizing UART I/O ports. It encapsulates the serial data stream into raw TCP/IP packets and delivers it to the remote IP address and socket associated with the network printer.

  2. Moxa NPort DOS Real TTY / Port Redirector: Moxa provides dedicated DOS drivers for its NPort device server hardware. This driver hooks standard COM port base I/O addresses (such as 0x3F8 for COM1) and IRQs, redirecting outgoing printer streams over the local network stack to an IP-attached serial printer.

  3. Digi RealPort for DOS: Similar to Lantronix and Moxa, Digi International produced a DOS TSR client that establishes a virtual COM port over the network, redirecting serial calls to network-connected serial hubs and printers.

  4. Novell NASI (NetWare Asynchronous Services Interface): In NetWare environments, serial traffic can be routed across the network using Novell’s NASI.EXE client driver. When paired with asynchronous communication servers on NetWare, it redirects INT 14h BIOS calls from the local workstation to network-attached serial devices.

Bridging COM to LPT for Standard Network Printing

If proprietary serial device server drivers are not an option, the standard method for routing serial printing to a networked printer share in MS-DOS 6.22 is using an intermediate COM-to-LPT software redirector:

  1. Software Port Re-mapping: A TSR utility (such as MODE.COM in reverse configurations, or third-party utilities like PORT2LPT or COM-LPT) intercepts communications directed at COM1 or COM2 and routes them to an unused parallel port (such as LPT1 or LPT2).
  2. SMB Print Redirection: The Microsoft Network Client driver (REDIR.EXE) then captures the data passing through the virtualized parallel port via NET USE LPTx: \\SERVER\PRINTER and transmits it across the local network to the shared printer queue.