---
name: "ast"
section: "4"
platform: "openbsd"
package: "man"
version: "8.0-current-20261101"
release: "OpenBSD 8.0-current (2026-11-01)"
platforms: ["linux", "openbsd"]
url: "https://new.carta.tech/ast/4"
---

# ast(4)

multiplexing serial communications interface

## NAME

**ast** – multiplexing serial communications interface

## SYNOPSIS

**ast0 at isa? port 0x1a0 irq 5** **ast1 at isa? port 0x2a0 irq 6** **com\* at ast?**

## DESCRIPTION

The **ast** driver provides support for boards that multiplex together up to four EIA RS-232C (CCITT V.28) communications interfaces. Apparently the original master of hardware using this multiplexing protocol was AST.

Each **ast** device is the master device for up to four **com** devices. The kernel configuration specifies these **com** devices as slave devices of the **ast** device, as shown in SYNOPSIS. The slave ID given for each **com** device determines which bit in the interrupt multiplexing register is tested to find interrupts for that device. The port specification for the **ast** device is used to compute the base addresses for the **com** subdevices and the port for the interrupt multiplexing register.

On a real AST card, the jumpers are as follows:

- SW1-1

  Turn ON for irq 2. Default is OFF.

- SW1-2

  Turn ON for irq 3. Default is OFF.

- SW1-3

  Turn ON for irq 4. Default is OFF.

- SW2-1

  Turn ON for irq 5. This is ON for the first card, OFF otherwise.

- SW2-2

  Turn ON for irq 6. This is ON for the second card, OFF otherwise.

- SW2-3

  Turn ON for irq 7. Default is OFF.

- SW3-1

  “Compatible mode”. The **ast** driver needs extended mode, so leave this OFF.

- SW3-2

  I/O Address. In extended mode, this should be OFF for the first card at 0x1a0-0x1a7 and ON for the second card at 0x2a0-0x2a7.

- SW3-3

  Interrupt sharing. Default is OFF.

- SW3-4

  Reserved, must be OFF.

## FILES

- */dev/tty0?*

  ## SEE ALSO

[com(4)](/com/4.md), [intro(4)](/intro/4.md), [isa(4)](/isa/4.md)

## HISTORY

The **ast** driver was written by Roland McGrath and placed into the public domain.