RS-232 Tech Bulletin from sandman.com
RS-232 Serial Communications is NOT that hard! For the stuff
that the average phone man has to do, our Economy Model Break Out Box and
a little basic information should allow you to get printers, SMDR and
modems working.
If you are working with modems, please note that most modern modems use
the Hayes AT Command set. That means that you need to set up the modem
using a computer or terminal, then save that setup in a special memory
that stays with the power off. Please consult the modem manual for further
information.
Please remember that the numbers on the actual dip switches DON'T
line up with actual pin numbers, printed on the faceplate on some Breakout
Boxes. The holes for the jumpers DO line up with the printed
numbers on the faceplate.
ALWAYS USE THE NUMBERS PRINTED ON THE
FACEPLATE!
Pin 1 is a hardware ground that does not have a switch on our
Economy Breakout Box
, and is always shorted through to each plug. There are no sockets for
jumpers for Pin 1. YOU MUST FLIP SWITCH NUMBER 7 ON, for any
data to pass thru our Economy Box. Pin 7 is signal ground, and it
MUST
ALWAYS BE ON!!!
The lights on the side of the box refer to the pin numbers. Signals
from one direction will be positive (green), and from the other direction
will be negative (red), usually. With no signal, the LED is out. With
data, you should see pins 2 or 3 flickering, faster at the higher baud
rates. It may seem to be on steady above 4800 baud. If pins 2 and 3 need
to be reversed, you will often see the LEDs for those pins go out when you
plug in the second piece of equipment. This is because they have different
polarities, joining in the box. Just open the dip switches for 2 and 3,
and the LED will come back on. This tells you that pins 2 and 3 need to be
crossed (often referred to as a null modem). Just open the switches, and
use a supplied jumper to go from pin 2 on one side, to 3 on the other.
Then put another jumper in from pin 3 on the first side to pin 2 on the
other. Both lights should be lit, and you should see them change color or
flicker while data is passing thru. Some devices like printers, only receive
data - they won't transmit. In this case, you will only get a light on pin
2 or 3 from the other piece of equipment. This is really pretty easy!
HERE'S THE BIG SECRET: Pins 4, 5,
6, 8 and 20 are special pins that act like traffic cops, telling the
equipment when to send and when to stop sending data. Pin 8 is Carrier
Detect (CD), which usually is positive when a modem has detected a carrier
and a phone connection is established. This usually needs to be made high
if the equipment your using doesn't make it high. Bottom line, if you
short some combination of pins 4, 5, 6, 8 and 20 together, everything will
work fine at the lower baud rates (like 2400-4800). You may start to lose
data at 9600 baud if you short out the hardware flow control, but it may
still work OK if the equipment uses software flow control (often called X-On,
X-Off). Who knows... until you start playing with it, and you
really don't usually care as long as it works. Try shorting 4 and 5
together. Then try shorting 6 to 20 together. Then try shorting 8 to
either the 4-5 or 6-20 combination. Then try shorting them all together.
Then try a different combination. It might not sound scientific, but this
is the way to make the equipment work in the shortest amount of time.
Don't worry, I've never blown up a serial port doing this yet, but I'm
sure it can happen. I have blown up serial ports from static electricity.
If you're working on a carpet, use a ground strap to your wrist, or ground
yourself often. A blown port can be pretty expensive. As a tip, the chip
that controls the serial data is often called a UART (Universal
Asynchronous Receiver Transmitter). If this pin is socketed, you may be
able to replace that chip and be on your way.
Please keep in mind that software flow control (X-On, X-Off)
can sometimes fool you...even if your connections are correct the
equipment may be "locked up" due to software. You can try
resetting the equipment (turn it on and off), or try sending Control A,
Control S then Control A, Control X or Control Q from a keyboard, which
may release the software flow control. You are better off resetting the
equipment after making a change in connections.
DON'T LET SOFTWARE FLOW
CONTROL FOOL YOU!
To use our
Economy Breakout Box
, just plug it in. There are male and female
DB-25 connectors on the box, so it will plug in like an extension cord.
Just plug it in any way you can. Technically, it makes a difference as to
which side is DTE and which side is DCE (see chart on cover), but it
usually won't matter.
You can use the pins on either side to "daisy chain" pins
together, as long as the switch is thrown. There are also some pins
labeled "jumper," which you can use for the same purpose. There are two
extra pins at the top, where pin one would be, that allows you to use the
spare LEDs for pins that don't already need them, although you probably
will never use them.
I would use a beanie type crimp, or solder together 5 jumper wires on
one end (then insulate), since that is what you will need on most jobs,
and it's easier than daisy chaining them thru the box. I don't like the
jumpers that come with this Economy Box because they use stranded wire
that is "springy", and you can't fold them up and store them in
the box (although stranded will last longer than solid). I would make up a
bunch of jumper wires from solid copper phone type jumper wire, and carry
them folded in half in the case).
Once you have gotten the connection working, you need to make note of
which wires are reversed or shorted, and modify an existing cable or make
a new one with the connections according to your breakout box.
If you don't have a serial port on your PC, you could try our
USB to Serial Adapter
.
There is 90% chance it will work with a particular serial device, not 100%.
One word of advice... not all USB to Serial Adapters work well. A lot of
newer computers only have USB ports to save money, or space on a laptop. Most USB to Serial
Adapters will work OK for occasional use, but it's not a great idea to use
it for a permanent connection that must work if the PC is restarted. While
the drivers for a motherboard serial port seem to be rock solid, the
drivers written for the cheap USB to Serial Adapters (written in some third
world country) aren't so solid. The most likely time for them to fail is
when the PC reboots. Sometimes the drivers just don't load on boot-up, so
your serial gizmo won't work after an unattended reboot.
SERIAL COMMUNICATIONS REALLY ISN'T THAT HARD!
Also Contains:
EASY
INSTRUCTIONS FOR RS-232 BREAKOUT BOX
