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Hi Ryan,
The pin-out of the rs232 connectors depends on
whether the connector is wired for a DTE or DCE device. Some rs232 devices
are the computer, some are the peripheral device. Here is a link to a site
that has some good graphics and explanation. The text below the link is
from the site and explains the issue.
There is one other issue that comes to mind.
Some embedded devices draw power from the control lines to power the TX
line. I looked at some old posts in this group and saw that this was a
problem for some people. Here is a Message
from Onur T from 1/2006
Hi Mike, On my end I have had to build a small RS232
line driver circuit to talk to an embedded module which had CMOS outputs.
I could be mistaken but if the device you want to connect to is at CMOS
or low power TTL and does not have a driver circuit build, you may have to build
one( 1 chip, plexiboard, 1 week TV time ). A lot of times the embedded modules
do not necessarly have the line driver interface built in, and may just give the
TTL/CMOS outputs right from the chip. Also as a quick try, try a power
supply with more amperage. For me it got rid of a whole lot of problems just
changing the power supply ( I suspect my embedded module and TS-2000 were
drawing peak power at the same time and causing a reset on the TS-2000).
m g <> wrote:
On
Wed, Jan 18, 2006 at 07:28:05PM -0000, PeterElliot wrote:
> Another
issue is that some low power devices used a scavenging
> technique to
pull power from the unused lines to generate the voltage
> used for
their serial output.
>
> Have you tried putting a scope on the
output from the PDA to see if
> you are getting valid RS232
voltages?
Interesting. I had planned on hooking up a scope, just
haven't had the
time lately to work on this problem. I'll post what I
discover with the
scope.
I haven't worked with RS232 in a long time
but I'm sure it is supposed
to work at TTL levels. But, how about 3.3-V? Is
3.3-V all that is
available on the TS? Time to do some research. If this
indeed is the
problem, does anyone know of a solution? Am I the only one
trying to
connect a low power device to the
TS?
Thanks,
Mike
Is Your Interface a
DTE or a DCE?
One of the stickiest areas of confusion in datacom is over the terms
"transmit" and "receive" as they pertain to DTE (data terminal equipment) and
DCE (data communication equipment). In synchronous communication, this confusion
is particularly acute, because more signals are involved. So why is it that you
sometimes send data on TD, and other times you send data on RD? Is this just a
cruel form of mental torture? Not really. The secret lies in adopting the proper
perspective. In data-com, the proper perspective is always from the point of
view of the DTE. When you sit at a PC, terminal or workstation (DTE) and
transmit data to somewhere far away, you naturally do so on the TD (transmit
data) line. When your modem or CSU/DSU (DCE) receives this incoming data, it
receives the data on the TD line as well. Why? Because the only perspective that
counts in data-com is the perspective of the DTE. It does not matter that the
DCE thinks it is receiving data; the line is still called "TD". Conversely, when
the modem or CSU/DSU receives data from the outside world and sends it to the
DTE, it sends it on the RD line. Why? Because from the perspective of the DTE,
the data is being received! So when wondering, "Is this line TD or RD? Is it TC
or RC?" Ask yourself, "What would the DTE say?"
Find out by following these steps: The
point of reference for all signals is the terminal (or
PC).
1 ) Measure the DC
voltages between (DB25) pins 2 & 7 and between pins 3 & 7. Be sure the
black lead is connected to pin 7 (Signal Ground) and the red lead to whichever
pin you are measuring.
2) If the voltage on pin
2 is more negative than -3 Volts, then it is a DTE, otherwise it should be
near zero volts.
3) If the voltage on pin
3 is more negative than -3 Volts, then it is a DCE.
4) If both pins 2 & 3
have a voltage of at least 3 volts, then either you are measuring incorrectly,
or your device is not a standard EIA-232 device. Call technical
support.
5) In general, a DTE
provides a voltage on TD, RTS, & DTR, whereas a DCE provides voltage on
RD, CTS, DSR, & CD.
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