Modem (API)

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This page is for the modem API. For the blocks, see Modem.
This API requires the use of a wrapped peripheral!
This API does not behave like a regular API, to use it you must first wrap the peripheral and call the methods on the wrapped object. For more information see this page.


Available to wrapped Wireless, Ender, or Wired Modems.

Direct use of modems requires understanding of the "channels" system - these are essentially networks which can be opened, closed and listened on by any Computer within range, without need of independent computer IDs. To interact with channels, one must wrap or otherwise interact directly with the peripheral. The Rednet API acts as a wrapper for your modem's functionality, handling the channel system for you (basically by having each system listen on a channel equal to its ID number).

Functions in italics are only available to Wired Modems, which are able to connect to remote peripherals and control them from afar.


Grid disk.png  Modem (API)
Function Return values Description
modem.isOpen(number channel) boolean isChannelOpen Checks to see if channel is open.
modem.open(number channel) nil Opens channel to allow for listening. The channel specified must be larger than 0 and less than 65535.
modem.close(number channel) nil Closes an open channel to disallow listening.
modem.closeAll() nil Closes all open channels.
modem.transmit(number channel, number replyChannel, any message) nil Transmits a message on the specified channel.
modem.isWireless() boolean isWireless Returns if the modem is wireless or wired.
modem.getNamesRemote() table peripheralNames Returns a table containing the network names of the peripherals connected to the modem.
modem.getTypeRemote(string peripheralName) string type Returns the type of a given peripheral connected to the modem.
modem.isPresentRemote(string peripheralName) boolean present Returns whether a given peripheral is actively connected to the modem.
modem.getMethodsRemote(string peripheralName) table methodNames Returns a list strings naming the functions available to the specified peripheral.
modem.callRemote(string peripheralName, string method, ...) any Has the remote peripheral execute its specified function.


Grid Modem.png  Event modem_message
Fired when a modem message is received.
Returned Object 1 string modemSide
Returned Object 2 number senderChannel
Returned Object 3 number replyChannel
Returned Object 4 message
Returned Object 5 number distance


Sending Messages

Sending a message is simple and does not require that you open any channels. Simply use the transmit function like so:

local modem = peripheral.wrap("right") -- Wraps the modem on the right side.
modem.transmit(3, 1, "Hello world!") 

peripheral.call("right", "transmit", 3, 1, "This will also work!")

What did that do? First I wrapped the peripheral in order to interact with it. Second, I used modem.transmit(channel, replyChannel, message) in order to send my message. In case you were wondering, the reply channel is captured by the listening computer and suggests which channel they should reply on.

Receiving Messages

Receiving a message requires that you be familiar with events. An example:

local modem = peripheral.wrap("left")
modem.open(3)  -- Open channel 3 so that we can listen on it
local event, modemSide, senderChannel, 
  replyChannel, message, senderDistance = os.pullEvent("modem_message")

print("I just received a message on channel: "..senderChannel)
print("I should apparently reply on channel: "..replyChannel)
print("The modem receiving this is located on my "..modemSide.." side")
print("The message was: "..message)
print("The sender is: "..(senderDistance or "an unknown number of").." blocks away from me.")

So what did I do? Quite simply, I called os.pullEvent() with the string argument "modem_message", which blocks all other events from being returned. When the "modem_message" event is captured, it returns the arguments: event, modemSide, senderChannel, replyChannel, message, senderDistance. I captured these and then printed them out.

Modem Limitations

  • You can only open 128 channels at any given time, per modem.
  • The highest channel you can open is 65535.
  • You can listen on more than one channel at a time. For example, if the modem has channel 3 and channel 5 open, and somebody sends a message on channel 5, the modem will receive it. If a message is sent on channel 3, the modem will also receive the message.
  • Sending messages does not require you to open any channels prior to sending it.
  • If you aren't receiving a message when you think you should, check to make sure that you have opened the channel first.
  • Modems and channels are not secure - if you are sending a message using the Modem API, messages are still available to any computer listening on the sent channel. By extension, the Rednet API (which handles your modems for you using their peripheral APIs) is also insecure.