- Jan 1, 1970
[snip]I still think that you are trying to solve the wrong problem.
By using some add-hoc driver design, you are most likely going to
create much more compatibility issues with devices from various
Since the RS-485 standard requires a +/-200 mV swing between the
receiver terminal, with 54 ohm total load, the driver must be able to
supply _at_lest_ +/-3.7 mA.
With +/- 2V Tx voltage swing, the total loop resistance should be
below 540 ohms, i.e 240 ohms in a single conductor, so at least 0.3 mm
wire diameter is required for 1000 m.
Which reminds me... somewhere in the basement of my mind I remember
working on a bus driver which was CURRENT-drive.
Upsidedown's comments suggest doing just that plus adding a voltage
I usually try to analyze a point-to-point RS-422 circuit as a bipolar
current loop with at least +/- 1.7 mA loop current through the 120 ohm
receiver termination resistance. Compare this to the unipolar 0..20 mA
(or even 0..60 mA) Teletype era unipolar current loops. RS-485
multidrop circuits are a bit more complicated.
At higher data rates, the bus must be analyzed as a section of (more
or less) matched transmission line.