IrDA on 2.1 Devices (Part 3)
The most interesting parts of the IrDA stack are IrCOMM and IrOBEX. IrCOMM emulates a serial connection and IrOBEX facilitates exchange of chunks of data, similar to HTTP. IrCOMM and IrOBEX are usually implemented on top of TinyTP (there is one variant of IrCOMM that uses IrLMP directly).
The goal is to enable the Newton to use IrCOMM instead of one of the serial ports (built-in port, PCMCIA serial cards, PCMCIA modem cards) to connect to other IrCOMM enabled devices. Most interestingly, this would be cell phones which can be used as wireless modems. I don’t have any firm plans for IrOBEX, but we’ll see…
IrCOMM follows the same principle as most other communications protocols: Use an existing layer, build on top of that and put any specific control data into the data part of the underlying frame format. IrCOMM introduces a number of control structures which are mostly used for flow control. It can emulate basic serial three wire without flow control, three wire serial with XON/XOFF flow control, nine wire serial with hardware flow control, or Centronics flow control. The control parameters are put into the beginning of a TinyTP frame, just after the TinyTP credit byte. The structure is simple: first a byte that tells how big the control structure is, and then (parameter type, parameter length, parameter data)-tuples. All this is defined in the IrCOMM specification.
To make IrCOMM work on the Newton, we need to have a working TinyTP layer. As noted before, IrCOMM will most likely work only when initiating the connection. But that should be ok for our purposes. The IrCOMM layer will open a regular TinyTP session to the peer’s IrDA:IrCOMM service. The IrCOMM service we request will be nine wire serial with hardware flow control, but we’ll indicate that we won’t use flow control. Maybe at some point later it can be added, but for now, I’ll try to go without it.
The remainder of the work is to make IrCOMM available as a regular serial port. For that, we have to go under the hood of the Newton Communications Manager. Fun stuff ahead!