On 16/02/2018 13:03, dennis@home wrote:
On 16/02/2018 01:11, John Rumm wrote:
On 15/02/2018 18:18, Huge wrote:
On 2018-02-15, John Rumm wrote:
On 14/02/2018 11:46, jim wrote:

If the serials are in use Laplink used to use parallel ports?

https://en.m.wikipedia.org/wiki/LapLink_cable

The original laplink cable had both serial and parallel versions - they
did one with three connectors on each end - a DB25 male for parallel,
and DB25 & DB9 female for serial.

The parallel version did faster transfers (defaulting to a nyble wide
data path on a standard parallel port),

Really? IIRC (and I may well not) it worked by twiddling the status
lines, because most parallel ports only had unidirectional data lines,
and was therefore slower.

While there were plenty of output lines available, there were only 5
available inputs (error, ack, paper empty, busy, & Select), so the
parallel cable cross wired those to give 4 data lines, and a strobe in
each direction.

God, I miss all that stuff not at all!

It was pretty cool at the time - you could transfer a whole directory
of files with between machines with incompatible sizes/densities of
floppy disk very easily.



I had some that used bidirectional parallel ports and they were much
faster than serial ports.

The standard parallel port version of laplink could run about 4 to 5
times faster than the serial version IIRC. The serial one was limited to
115k2

One of the cards I designed for System X used a serial port running at
880kbits/sec. That was pretty fast at transferring data to a tape drive.
IIRC the same chip was used in quite a few PCs to provide the serial
ports so they had the potential to run very fast.

The UARTs themselves are usually somewhat more capable than the system
design would permit. It usually depends on how fast the UART was
clocked. Typically the bit rate would be 16th of the clock rate after
scaling by the UARTs clock divisor. The 8050A UART used in early PCs
could in theory do up to 625Kbps, but that required a 10Mhz clock. The
original PC design had a 1.8432 MHz clock, which gives you a max rate of
of 16th of the clock rate or 115K2.

Later devices like the 16550 with its built in FIFO, could take an
external clock up to 24MHz giving about 1.5Mbps maximum. However the
clock would normally be kept at the standard lower speed to maintain
software compatibility in PC applications.

(I remember working on one project that used a standard 8252A UART (not
dissimilar from the standard PC device - but it could be clocked at
16MHz)) fed from a hardware fifo, and that would reliably encode data at
1Mbps. We used it to encode telemetry data into a live video stream onto
some "spare" video lines. (i.e. same encoding technique as used for
teletext)



--
Cheers,

John.

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