1 Feb
2025
1 Feb
'25
9:55 a.m.
I wouldn't call them 'adjusted' to TTL levels. They're simply the levels
that are used between UART and line buffers, which, yes, are TTL but not
something arranged particularly for these devices. Of course, they aren't
any more suitable for external connections than any other internal
board-level signal. It's a hack, useful for making a short connection but
it's the USB part that's intended to be exposed externally.
Note that they're also inverted relative to the RS232 levels because line
buffers convert logic 1 to -15V.
Some USB-serial chips have the ability to choose whether the signals are
inverted. They also tolerate negative input voltages. This allows them to
interoperate with buffered lines which interpret 0v as a negative voltage,
but with extremely bad noise margins.
On Sat, Feb 1, 2025 at 8:48 AM Steve Lewis via cctalk <cctalk(a)classiccmp.org>
wrote:
I'm not sure what you mean by 'a "TTL" logic level serial port',
please
elaborate. Do you mean signalling used between the UART and line drivers
by any chance, such as with a serial connection made between UARTs
without
That confused me at first as well. As you concluded: yes, not a "classic"
(or "real" serial port), but something adjusted to TTL-logic levels.
Reading the async-expansion for the IBM 5110, it talks about -25 to +25V
(the original spec of RS232?). On the 1980 Color Computer 1, I noticed it
uses -12V to +12V for its RS232. Later in the 1990s, laptops wanted to
sip less less power, and I think RS-232 revisions allowed for as low as -3V
to +3V swings? So those -5/+5V or 3.3 integrations get referred to as
more modern "TTL logic level serial port" (such as generally a USB/serial
adapter) to contrast from prior legacy devices.
But I suppose it means these devices have less range (max cable distance)
than the original spec? I don't recall all the specifics of the original
SAGE, but I think it was an array of IBM704s across buildings. Or at least
across floors of a building. So the gobs of voltage maybe made sense back
then. If P=IV, the using less Voltage should conserve some power, while
also being faster to "swing" (transition +/-) and allow greater speeds (but
at more limited range).
On Fri, Jan 31, 2025 at 9:37 PM Maciej W. Rozycki via cctalk <
cctalk(a)classiccmp.org> wrote:
On Fri, 31 Jan 2025, Paul Koning wrote:
> > FWIW I was able to get reliable serial communication under Linux of
up
> to
> > > 3.5Mbps with off-the-shelf proper PCIe UART hardware clocked at
> 62.5MHz
> > > despite that line drivers used with said hardware (soldered onboard)
> were
> > > spec'd for up to 1MHz only[1]. This was with plain PIO
> interrupt-driven
> > > operation, but then the UARTs used had decent FIFO sizes of 128
> characters
> > > and the FIFO trigger level for the interrupt was reasonably set.
> > >
> > > Finally at 4.0Mbps data corruption reproducibly triggered, but it was
> > > garbled rather than lost characters, so I conclude the reason was
> either
> > > line drivers finally giving up or the transmission frequency hitting
> the
> > > bandwidth limit of the serial communication cable used.
> >
> > Was that with an actual RS232 port, i.e., a device using RS232 signal
> > levels, or a "TTL" logic level serial port? I'm guessing the
latter.
I'm not sure what you mean by 'a "TTL" logic level serial port',
please
elaborate. Do you mean signalling used between the UART and line drivers
by any chance, such as with a serial connection made between UARTs
without
actual line drivers in between?
I've only seen such serial connections between onboard devices, such as
a
SoC's onchip UART wired to an FTDI-like
device soldered next to it on the
PCB for a USB serial console, which seems an industry's recent workaround
with development hardware for the usual lack of serial ports with modern
general-purpose computers. I don't expect this to work very well over a
cable unless very short.
V.28/V.24
In my high speed experiments, I found that the
limit for RS232 data
rates comes from the relatively slow rise/fall times implemented in
RS232 drivers. If you have a port that uses logic levels the
transitions are likely to be much faster so loss of signal integrity
occurs only at much higher speeds. With the RS232 drivers I have used
(MAX3222), 250 kbps is roughly the upper limit.
The serial port hardware I refer to uses a UART wired to a Zywyn ZT3243F
line driver, which according to the manufacturer's datasheet signals at
±5V minimum transmitter voltage levels and accepts up to ±25V receiver
voltage levels and: "Meets or Exceeds the EIA/TIA-232F and CCITT
Specifications for VCC at +3.3V ±10% and +5V ±10%
Operations." While the
transmitter voltage levels are not the highest recognised by the standard
I do believe this line driver does comply with RS-232.
As I say the datasheet explicitly says: "Guaranteed data rate 1000kbps,"
and according to my findings quoted above it is indeed the case (and well
beyond). [Yes, I got it wrong by writing 1MHz rather than 1Mbps, a
mental
slip I suppose.]
NB I've also used the TI TRS3122E line driver, suitable for operation
with 1.8V signalling per my requirement, and it is also documented to
handle "data rates up to 1000kbps, while maintaining RS-232-compatible
output levels." I haven't got a chance to go beyond 230400bps with this
device though, but these two samples do suggest that supported operation
at 1Mbps isn't that uncommon for currently available RS-232 line drivers.
I've looked up the MAX3222 datasheet and it does say 250kbps max though;
I guess it's older technology then?
Does this answer your question?
Maciej