The EEPROM on the DUART28 adapter board has two valid configurations: DUART28C
and DUART28S.  As of this writing the S configuration is the default shipping
one, but this situation may change in the future.  The difference between the
two configs is in the USB VID:PID presented by the USB device, and this USB ID
difference has the following practical impact:

* The C configuration presents a custom USB ID and requires a custom patch to
  the Linux kernel ftdi_sio driver in order to work - without this ftdi_sio
  driver patch it won't work at all.  But if you do go through the pain of
  applying the needed patch to your Linux kernel ftdi_sio driver, the reward
  is that you get not only the two Calypso UARTs, but also working boot control
  outputs.

* The S configuration presents the default FT2232x USB ID and is therefore
  supported out of the box by the standard Linux ftdi_sio driver without needing
  any patches.  However, the adapter's CTL1 and CTL2 outputs cannot be used in
  this configuration (they will be bogusly asserted whenever Channel B ttyUSB
  device is opened), and thus they must be left unconnected, and you don't get
  to play with the remote boot control feature.

The physical hardware is identical between the two configurations, only the
EEPROM programming changes, thus end users need to be able to switch freely
between the two EEPROM configs as they wish.  This article explains how to
program the EEPROM back and forth between the two configs.

Using fc-duart28-conf utility
=============================

The present fc-usbser-tools package includes a specialized high-level utility
for examining and changing DUART28 adapter configuration.  This utility has two
operation modes:

fc-duart28-conf show		-- check and show current configuration
fc-duart28-conf set C|S		-- set new config by programming the EEPROM

The show command is completely non-invasive: it looks through the system tree
of connected USB devices, looking for a DUART28 adapter (as identified not only
by USB VID:PID, but also by manufacturer and product ID strings), and if it
finds a DUART28 device, the tool reports its current configuration.

The set command is invasive by necessity, as the fundamental identity of the
connected USB device is being changed: the tool commands the Linux kernel to
unbind the ftdi_sio driver from both interfaces (both ttyUSB devices disappear),
checks the current EEPROM configuration, and then reprograms it for the new
desired config.  Once the operation completes, the DUART28 adapter needs to be
unplugged and replugged for the EEPROM change to take effect and for the
ftdi_sio driver to rebind.

Unless you intentionally fool it by programming DUART28C or DUART28S EEPROM
config into some other FTDI device, fc-duart28-conf cannot hit a non-DUART28
USB device by mistake: VID:PID, manufacturer ID string and product ID string
all have to match either DUART28C or DUART28S.  If you have multiple DUART28
adapters plugged in, the tool will hit the first one it finds - not a
recommended arrangement, but also one which is not expected to arise by
accident.

Manual programming with ftee-gen2232c and fteeprom-prog
=======================================================

In order to be safe in terms of not accidentally hitting some other FTDI-based
USB device, fc-duart28-conf requires the current state of the DUART28 adapter
to be either valid DUART28C or valid DUART28S.  Under ordinary expected usage
this requirement is not a problem, but if you somehow manage to totally hose
your adapter's EEPROM, then fc-duart28-conf won't be able to recover it.  If
you find yourself in the latter situation, see DUART28-EEPROM-manual article
for the older manual method which is always possible.