---
name: "umcs"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd", "openbsd"]
url: "https://new.carta.tech/umcs"
---

# umcs(4)

ASIX/MosChip MCS7840/MCS7820 USB to serial UART driver

## NAME

**umcs** – ASIX/MosChip MCS7840/MCS7820 USB to serial UART driver

## SYNOPSIS

**device** **usb** **device** **ucom** **device** **umcs**

In [loader.conf(5)](/loader.conf/5.md): **umcs\_load="YES"**

## DESCRIPTION

The **umcs** driver provides support for various multiport serial adapters based on the ASIX/MosChip MCS7840 and MCS7820 chips. They are 2- or 4-port adapters with full-featured 16550-compatible UARTs and very flexible baud generators. Also, these chips support RS422/RS485 and IrDA operations.

The device is accessed through the [ucom(4)](/ucom/4.md) driver which makes it behave like a [tty(4)](/tty/4.md).

Different ports on device are presented as sub-units, like */dev/ttyU0.1* and */dev/ttyU0.2*.

## HARDWARE

The **umcs** driver supports the following USB to serial UART controllers based on the ASIX/MosChip MCS7840 and MCS7820:

- ST Lab U-360 two-port serial USB adapter

- ST Lab U-400 four-port serial USB adapter

## FILES

- */dev/ttyU\*.\**

  for callin ports

- */dev/ttyU\*.\*.init*

  - */dev/ttyU\*.\*.lock*

  corresponding callin initial-state and lock-state devices

- */dev/cuaU\*.\**

  for callout ports

- */dev/cuaU\*.\*.init*

  - */dev/cuaU\*.\*.lock*

  corresponding callout initial-state and lock-state devices

## SEE ALSO

[tty(4)](/tty/4.md), [ucom(4)](/ucom/4.md), [usb(4)](/usb/4.md)

## HISTORY

The **umcs** driver appeared in ports since December of 2010.

## AUTHORS

The **umcs** driver was written by Lev Serebryakov ⟨*lev@FreeBSD.org*⟩.

## BUGS

This driver doesn't support access to any fine tunes of chip, like RS522/RS485 mode, non-standard baudrates, etc.