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IrCOMM - Serial Communication

Concepts

The IrCOMM layer uses the IrLMP/IrTTP and IrLAP layers to offer serial data and control services by simulating RS232 over IrDA.

As for RS232, IrCOMM flow control between two nodes may use XON/XOFF, RTS/CTS lines or none.

Services

IrCOMM supports these services:

  • Control services
  • Data transfer services

IrCOMM services add a 2-byte overhead to the IrLMP frame with the appending of the IrTTP credit and IrCOMM control channel length after the IrLMP header.

There are four flavours of IrCOMM:

  • 3-wire raw
  • 3-wire cooked
  • 9-wire cooked
  • Centronics

Briefly, 3-wire raw connects directly to IrLMP, ie does not use the IrTTP layer, uses a different service name for IAS lookup and is data only, ie no control channel. 3-wire cooked is a subset of 9-wire cooked. Centronics is a simulation of a printer interface.

For 9-wire cooked, as well as supporting XON/XOFF flow control, RTS/CTS flow control is also supported.

Connection and disconnection services are handled by the control channel. An IrCOMM connect occurs where the first IrCOMM frame immediately after the IrLMP/IrTTP connect contains the IrCOMM link parameters and current DTE/DCE control line status.

As for IrLMP/IrTTP, all IrCOMM frames are I frames. There are three IrCOMM frame types:

  • control-only
  • control-and-data
  • data-only

The IrCOMM frame type is indicated by the control channel length in the IrCOMM header field.

Service 1 of 2 - Control Info

The control channel contains link parameters and flow control information.

For an IrCOMM frame, the control channel has precedence over the data channel, ie any control data must be processed first.

The control channel serves as the IrCOMM connect which then allows payload data to be transferred. As part of the IrCOMM connect, an initial credit value is specified.

As for RS232, there are several IrCOMM link parameters among which are:

  • Data rate
  • Data format
  • Flow control

These parameters are independent of the IrLAP parameters, eg for most mobile phones, it is usual for the IrLAP baud rate to be 9600 but the IrCOMM baud rate to be 19200.

Service 2 of 2 - Data Transfer

The data channel passes payload serial data.

Microlite-IrDA Notes

The simplest possible control channel scheme is used. The first IrCOMM frame is a control-only frame with:

  • The complete set of link parameters
  • No flow control is specified
  • All DTE control lines are set active

On any subsequent IrCOMM frame, if there is any control info received, (a control-only or control-and-data frame), control info is transmitted with:

  • All DTE control lines are set active

Thus, the remote stack is always enabled to transmit data without being restricted by the IrCOMM flow control.

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