[time-nuts] RS232 / GPS interface/prototyping board

Attila Kinali attila at kinali.ch
Tue Jun 14 15:49:17 EDT 2016

On Tue, 14 Jun 2016 18:39:35 +0200
Herbert Poetzl <herbert at 13thfloor.at> wrote:

> > I am not aware of any USB 2 UART devices (that doesn’t
> > necessarily mean much). They just don’t need to be that fast.
> All modern FTDIs are USB 2.0 (for example the FT232R,
> FT2232H and FT4232H) because they also allow for other
> types of data transfer besides the UART which require
> USB 2.0.

This is not completely correct. The main difference between
USB1.1 and USB2.0 are (small) changes in the descriptor tables
and the introduction of HighSpeed transfer speed (aka HS aka 480Mbit/s,
or what most people call USB2.0). No additonal data transfer type was
added, as the three available (Bulk, Interrupt and Isochronus)
were sufficient. None of these changes have anything to
do with what kind of device types (UART is just a device type)
are possible. There are plenty of UARTS that are USB1.1.

Most devices these days will be USB2.0 conform. Meaning that
their descriptor tables will follow the changed layout from
the new standard. It does not mean that they are HS devices.
Actually most I have encountered are still FullSpeed (aka FS aka 12Mbit/s)
devices. There are some HS UART's though, FTDI selling some of them.

The maximum transfer speed a HS device can achieve with
bulk transfers is 35MByte/s (due to overhead in the protocol
and reserved time slots). But only if the devices is done in
an intelligent way and is able to transfer the data in blocks
of 512byte. If smaller blocks are used, then the data rate
scales down accordingly.

Of the devices you mentioned above, all are USB2.0, but only
the FT2232H and the FT4232H are HS, the FT232R is FS only.
Ie, while the FT2232H and the FT4232H support transfer speeds
up to 12MByte/s, the FT232R supports only 3Mbyte/s
(Rule of thumb: if the FTDI serial chip's name doesn't contain
a H, it's most likely not a HS device)

			Attila Kinali

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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