This file, and all files referenced below and
contained in this distribution, are copyright (c) 1999
Bob Drzyzgula, bob@drzyzgula.org.

These are the CAD and CAM files for a Serial RS-232 to
RS-485 converter board. The board was designed using
Cadsoft Eagle 3.55r3.

File Contents are as follows:

seradp2.brd	Eagle PCB Layout
seradp2.cmp	Gerber top/componant-side copper
seradp2.drd	Excellon Drill File
seradp2.dri	Drill Information File
seradp2.drl	Excellon Drill definition file
seradp2.erc	Electrical Rule Check output
seradp2.gpi	Gerber information file
seradp2.plc	Gerber top/componant-side silkscreen file
seradp2.sch	Eagle Schematic File
seradp2.sol	Gerber bottom/solder-side copper
seradp2.stc	Gerber top/componant-side solder stop mask
seradp2.sts	Gerber bottom/solder-side solder stop mask
seradp2.whl	Gerber apeture wheel file
sa2sch.ps	Postscript printout of schematic
sa2brd.ps	Postscript printout of integrated board
readme.txt	This file

A viewer/editor for the Eagle-format schematic and layout files may
be obtained from Cadsoft, http://www.cadsoft.de.

You are welcome to do whatever you want with this design,
except (a) claim that it is yours, or (b) assume that it
will work as drawn; while I have sucessfully tested boards
made from these files, there is no assurance that they
will work for you. Thus, these files are offered with
NO WARRANTY as to the fitness for any purpose whatsoever,
and I cannot promise to help you with them if they don't
work.

That being said, here's a few quick operational notes:

JP1 controls where power comes from and goes to. I made my
uC board so that it delivers +5V to pin SV2:7. If you jumper
JP1:5-6 and leave JP1:1-2 and JP1:3-4 open, that's how the
board will get power. You can open JP1:5-6 and supply +5V to
pin JP1:4 and GND to pin JP1:1 (I didn't put them across
from each other because I was afraid that someone might
short +5V to ground), then you can power the board from an
external power supply. Jumpering all three connections on
JP1 will allow you to power the converter from an external
source *and* provide +5V out to the RS-485 network.

JP2 controls the use of transmission control on the RS-485
chip. In my application, this board will usually be wide
open all the time; the PC software doesn't even try to do
transmission control. Therefore, I will jumper JP2:3-4 and
JP2:5-6; this will hold the RS-232 chip quiet on the R2
pins (this probably isn't necessary because they's a
pull-down inside the 232 as well, but, well, I just didn't
like leaving it unconnected -- write it off as the overkill
of the inexperienced :-), and will strap the RS-485
transmitter so that the driver is enabled full time. If you
instead jumper JP2:1-2 and JP2:7-8 and leave JP2:3-4 and
JP2:5-6 open, then RTS (pin SV1:4) from the PC will be
delivered to the DE control via the R2 circuit on the RS-232
chip. If you have PC software that expects this behavior
(not common, I don't think, on a full-duplex net), then you
can jumper it this way.

Since I ran out of receivers on the 232 chip I couldn't (and
it wouldn't have done me any good anyway), but the DTR on the
PC side could also be used to control RE/ in the same manner.
Alternatively, one could put an inverting buffer (say a 74xx04)
in the circuit and control RE/ and DE from RTS, sort of like a
connected/not-connected control.

If you don't need any sort of transmission control on the
485 side, you could of course use a tranceiever without any
such controls. If you need to do half duplex, though, you'll
probably need to do something like this, and rethink a
little how the circuit is wired up.

Finally, note that there is a 120 Ohm termination resistor on
the receive side of the RS-485. Depending on your transmission
medium, 120 Ohm may not be appropriate. Also, there is no
provision to terminate the transmit side of the RS-485, since
in my application this converter board is the only device that
ever transmits on that side, and thus termination on the
other end is sufficient.

The parts I used included the Maxim MAX489CPD, the Texas
Instruments MAX232N, generic 5%, 1/4W resistors for all
but the 120 Ohm, which was 5% 1/2W, a generic 2V, low-current
LED, and Panasonic EF Series Tantalum Capacitors (all 1uF).

Good luck,
--Bob Drzyzgula
bob@drzyzgula.org