17 Jun
2013
17 Jun
'13
5:22 p.m.
On Jun 17, 2013, at 10:57 AM, Guy Sotomayor <ggs at shiresoft.com> wrote:
Is it because of baud (bit) rate or an incompatibility?
Often times, it's because of drivers. Sometimes it's because the baud
generators on the chip won't go that low, or at least it's not called
out in the spec sheet as supported (FTDI's datasheet for the FT2232,
for example, only calls out as low as 300 baud). Other times, it's
because the driver writers never bothered to implement support for
anything that low ("Why would anyone want to go THAT SLOW these days,
anyway?").
In the case of hardware support, a lot of stuff that's mostly only
specific to Teletypes (5-bit characters, 1.5 stop bits) sometimes gets
eliminated because "no one uses it anyway", and I feel like sometimes
110 baud gets incorrectly lumped in with that set.
Only one problem there, most (if not all) EIA-232
adapters for USB won't
go below 300 Baud. It is a problem when trying to get the old ASR33 to
type things out!
Of course you need to also add two stop bits, but that is pretty easy (as
he notes the only place two stop bits were used was for the 33/35
teletypes).
I've experienced this, trying to use a USB serial adapter in place of an
ASR-33 to serve up an imaged paper tape on a PDP-8/e. It does not work.
It's an interesting question as to why this is a problem. :-) If the former, most baud rate generators won't
generate 110 unless specific
crystals/divisors are chosen. Otherwise they'll start at 300.
Generally, divisors aren't that hard (unless you run out of bits).
Some of the USB converter chips have a selected set of divisors in the
chip as a lookup table, though (they assume you're running with one
fixed crystal speed, which you need for USB anyway). Even if you
don't have an exact division factor (which you won't for getting 110
baud out of a 48 MHz oscillator, which is somewhat common for USB),
as long as it's off by only a few percent, you're not likely to see
any adverse effects on the other end.
If the latter, I've found that some USB serial
adapters aren't friendly to classic
gear. I haven't thrown an analyzer between them to figure it out. I've found
some that seem to work most often (Belkin springs to mind...still doesn't
support 110 though).
It's usually down to their RS232 level shifters. PC hardware is
usually pretty accommodating of less-than-standard levels, partly
because PCs themselves (especially older laptops) have been notorious
for producing somewhat less-than-standard voltages, but a lot of
vintage gear really does want you to get pretty close to +/- 12v.
The converters with good level shifters work fine, but that is also
unfortunately totally independent of the USB chip used, so you can't
really tell without either opening one up or using a scope. The sad
thing is that a good level shifter still costs less than a buck, but
to a company producing 10 million items, the difference between "less
than a buck" and "just a few cents" is astronomical.
- Dave