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modem

Summary

This page documents the wired modem component exposed by the standard OpenComputers network card. It lets a machine open ports, check port state, send packets to a specific address, and broadcast packets across the reachable wired network.

In generated docs this often appears as opencomputers-network-card, but the actual component name visible to Lua is modem.

Availability

  • Repository: opencomputers
  • Lua component name: modem
  • Typical host: wired network card, rack network interface, devices exposing the wired modem implementation

Important Notes

  • Valid port numbers are 1..65535.
  • Opening too many ports raises "too many open ports".
  • Ports are cleared when the attached computer starts or stops.
  • This wired card reports isWireless() == false and isWired() == true.

Usage

local component = require("component")
local modem = component.modem

if not modem then
  error("modem component not available")
end

Receiving Packets

Incoming packets arrive as modem_message signals when the destination port is open on this card.

Typical event payload:

"modem_message", receiverAddress, senderAddress, port, distance, ...

On a purely wired modem, distance is typically 0.

API

open

  • Syntax: modem.open(port)
  • Returns: boolean
  • Purpose: Opens one local port for incoming packets.

Parameter:

  • port:number: port number in the range 1..65535.

Return value details:

  • true when the port was newly opened,
  • false when it was already open.

Possible error from source: "too many open ports".

close

  • Syntax:
    • modem.close()
    • modem.close(port)
  • Returns: boolean
  • Purpose: Closes one port, or all open ports when called without arguments.

Return value details:

  • without an argument: true if any ports were open and got cleared,
  • with a port: true if that port was open and got closed.

isOpen

  • Syntax: modem.isOpen(port)
  • Returns: boolean
  • Purpose: Checks whether a local port is currently open.

isWireless

  • Syntax: modem.isWireless()
  • Returns: boolean
  • Purpose: Reports whether this modem supports wireless transmission.

For the wired card described on this page, this returns false.

isWired

  • Syntax: modem.isWired()
  • Returns: boolean
  • Purpose: Reports whether this modem supports wired network traffic.

For the wired card described on this page, this returns true.

send

  • Syntax: modem.send(address, port, data...)
  • Returns: boolean
  • Purpose: Sends a packet to a specific target node address.

Parameters:

  • address:string: destination node address.
  • port:number: destination port in the range 1..65535.
  • data...: payload values.

Behavior:

  • The packet is transmitted across the reachable wired network or rack-neighbor network, depending on host type.
  • The call returns true after queueing the send operation.

broadcast

  • Syntax: modem.broadcast(port, data...)
  • Returns: boolean
  • Purpose: Sends a packet to every reachable receiver listening on the specified port.

Parameters:

  • port:number: destination port in the range 1..65535.
  • data...: payload values.

This is the usual way to implement discovery protocols or simple LAN chat.

maxPacketSize

  • Syntax: modem.maxPacketSize()
  • Returns: number
  • Purpose: Gets the configured maximum packet size.

This is the hard upper bound for one packet's payload encoding.

Example

local event = require("event")

modem.open(1234)
modem.broadcast(1234, "hello", os.time())

local _, _, from, port, _, message = event.pull("modem_message")
print("From:", from, "Port:", port, "Message:", message)

Related

  • wireless modem
  • tunnel
  • relay
  • component