15 Oct
2015
15 Oct
'15
11:42 p.m.
On 10/15/2015 10:58 PM, Brent Hilpert wrote:
But working from OSX unix system-call level to a
plastic-encapsulated
USB-serial dongle, I was pleasantly surprised that "75" was accepted
by the IO/device driver. IIRC, it didn't accept 110 or other values
outside the divide-by-2 sequence, and 75 was the final. So it wasn't
a general frequency-to-period calculation from a high-res timer.
Maybe it was a factor-of-two calculation that hit the counter
resolution limit at 75, or the driver writer actually included 75 in
a table lookup.
All USB-to-RS232 bridge chips appear to derive their BRG base from a
12MHz internal oscillator. In spite of some folks' high opinion of FTDI
chips, they're far from the most flexible. The FT231x, for example,
bottoms out at 183.1 bps.
On the other hand, the rather "prolific" Prolific PL2303, found on many
cheap Chinese adapters does, in fact, bottom out at 75 bps with the
standard driver. Apparently, custom/nonstandard rates are also
possible, but the datasheet specifies that Prolific needs to be
contacted for details. I suspect that this involves an NDA.
The Nanjing Qin Heng CH340, also found on inexpensive adapters, reached
down to 50 bps.
The Microchip MCP2200 appears to bottom out at 300 bps as well, but
there's a configuration utility available (according to the datasheet)
that allows for non-standard rates. No mention of the minimum, however
At first blush, it seems that cheap Chinese converters could be your
friend in this case.
The USB-to-TTL dongles that I pay around $1 for shipped from eBay use
the PL2303. Perhaps one could form the basis of a current loop to USB
interface.
FWIW
Chuck