31 Jan
2025
31 Jan
'25
7:37 p.m.
On Fri, 31 Jan 2025, Paul Koning 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
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.
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.
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 V.28/V.24
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