On 2014-01-27 12:43, Javier Herrero wrote:
On 27.01.2014 19:33, Robert Atkinson wrote:
All the receiver chips I've looked at, ancient and modern, have only positive
thresholds. Most have single supplies and clamp the input at 1 diode drop
negative WRT common after an input current limiting resistor, see the MC1489
datasheet.
Not exactly. If you check the MC1489 datasheet from On Semiconductor, the
thresholds can be programmed with the response control resistor and can in fact
be negative. (Figures 6 and 7 in the datasheet). The serial input resistor
forms part of a resistive divider with the feedback resistor and the external
resistor - not simply a current limiter to the diode.
Recent device app notes state the spec is more or less the same,
with TIA-232-F making some changes to conform to current versions
of ITU V.24, V.28, and ISO 2110, restating the old speed/distance
curves in terms of load capacitance < 2500pF; output is still max
+/-25V @ 100mA, rise/fall time 4% of bit time, up to 30V/us slew
out of 300ohm impedance, input min +/-3V into 3k-7kohm < 2500pF.
Most recent adapters will work with only 0/3.3V over short cables
at low speeds: higher speeds up to 115,200bpps require more drive
and are limited to 3m, low speeds may work over 10m.
Good luck finding any docs for these interface boards nowadays.
YMMV should have been stamped on these specs since day one ;^>
--
Take care. Thanks, Brian Inglis
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