Author Topic: connecting tx/rx to an rs232>usb converter  (Read 7716 times)

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Offline dequo

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connecting tx/rx to an rs232>usb converter
« on: October 05, 2011, 07:38:00 am »
I have an angle sensing instrument that communicates with rs232 protocol to a com port on the computer via http://australia.rs-online.com/web/p/products/0550170/ , i am trying to read it on the com port but im not getting anything at all.
the device has 4 wires - power, ground, RX, TX

the TX from the device is connected to the RX in the DB9 RS232 connector (pin 2)
RX connected to TX (pin 3)
Ground connected to ground (pin 5), also connected to power supply ground
power connected to power supply ground

i have set the specified port options in the device configuration (115200 baus, start bit 1, stop bit 1, data bit 8,no parity checks, no flow control)

anything im doing obviously wrong? do i need to do something with all the DB9 pins other than TX, RX and GND?
 

Offline IanB

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Re: connecting tx/rx to an rs232>usb converter
« Reply #1 on: October 05, 2011, 07:50:41 am »
anything im doing obviously wrong? do i need to do something with all the DB9 pins other than TX, RX and GND?
It's possible. You need to the exact specs and connection requirements of the device you are communicating with. Sometimes it might be required to connect other signals like DTR to enable communication. If in doubt you should use a fully wired cable with all pins connected. You might also check voltages. I think "officially" you need 12 V signals for RS-232, though a lot of hardware is forgiving in this respect.
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Offline Bored@Work

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Re: connecting tx/rx to an rs232>usb converter
« Reply #2 on: October 05, 2011, 07:59:36 am »
Did you really pay 60 AUD for that adapter? Because that is 10x more than normal.

And are you sure that you really have an instrument using RS232 voltage levels, as opposite to 5 V or 3.3 V logic levels? The power wire is rather suspicious, because it is uncommon (i.e. non existing) on real RS232 interfaces.
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Offline desowin

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Re: connecting tx/rx to an rs232>usb converter
« Reply #3 on: October 05, 2011, 08:10:52 am »
the device has 4 wires - power, ground, RX, TX

What is this device?
Looks like it's just plain UART on TTL levels, not RS232 levels (positive or negative voltages in range 3-15V). For that you'd need (assuming you want to connect to USB on computer side) something like Microchip MCP2200 or FTDI FT232.

anything im doing obviously wrong? do i need to do something with all the DB9 pins other than TX, RX and GND?

Well, if target device operates on TTL levels, then it's likely that you are close to (or already did it) frying it.
« Last Edit: October 05, 2011, 08:15:46 am by desowin »
 

Offline dequo

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Re: connecting tx/rx to an rs232>usb converter
« Reply #4 on: October 05, 2011, 08:15:00 am »
ok, either its labelled wrong or im an idiot. connecting what is apparently TX on one to the TX of the other works...
 

Offline Psi

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Re: connecting tx/rx to an rs232>usb converter
« Reply #5 on: October 05, 2011, 08:17:28 am »
ok, either its labelled wrong or im an idiot. connecting what is apparently TX on one to the TX of the other works...
heh it's labeled wrong,
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Offline IanB

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Re: connecting tx/rx to an rs232>usb converter
« Reply #6 on: October 05, 2011, 10:22:20 am »
ok, either its labelled wrong or im an idiot. connecting what is apparently TX on one to the TX of the other works...
heh it's labeled wrong,
Why would it be labeled wrong? Connecting TX to TX and RX to RX is exactly what you are meant to do in the normal case where one device (computer) is a DTE and the other (peripheral) a DCE.
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Offline Psi

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Re: connecting tx/rx to an rs232>usb converter
« Reply #7 on: October 05, 2011, 12:24:52 pm »
ok, either its labelled wrong or im an idiot. connecting what is apparently TX on one to the TX of the other works...
heh it's labeled wrong,
Why would it be labeled wrong? Connecting TX to TX and RX to RX is exactly what you are meant to do in the normal case where one device (computer) is a DTE and the other (peripheral) a DCE.

Doesn't matter, it's still wrong.

If the world says you should connect the transmit pin of one device to the transmit pin of another device then the world is wrong.

I'm making a stand, who's with me?
« Last Edit: October 05, 2011, 12:30:26 pm by Psi »
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Offline Chet T16

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Re: connecting tx/rx to an rs232>usb converter
« Reply #8 on: October 05, 2011, 12:50:50 pm »


Doesn't matter, it's still wrong.

If the world says you should connect the transmit pin of one device to the transmit pin of another device then the world is wrong.

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Offline metalphreak

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Re: connecting tx/rx to an rs232>usb converter
« Reply #9 on: October 06, 2011, 12:23:06 am »
The silkscreen/label can have two meanings  :P

"Connect to these pins on YOUR device"

OR

"These are the pins on THIS device"

Personally, the second one makes more intuitive sense to me since its usually some kind of breakout board. They should use something like HTX/HRX and TTX/TRX for Host/Target labeling.

Offline Hypernova

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Re: connecting tx/rx to an rs232>usb converter
« Reply #10 on: October 07, 2011, 02:50:15 pm »
One reason why SPI use MISO/MOSI (Master IN Slave OUT)  :)
 

Offline Psi

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Re: connecting tx/rx to an rs232>usb converter
« Reply #11 on: October 07, 2011, 11:52:34 pm »
One reason why SPI use MISO/MOSI (Master IN Slave OUT)  :)

yeah,  SPI does it the right way.
It's obvious what it means and how you should connect it.
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Offline IanB

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Re: connecting tx/rx to an rs232>usb converter
« Reply #12 on: October 08, 2011, 12:03:38 am »
yeah,  SPI does it the right way.
It's obvious what it means and how you should connect it.
But look at the top two pictures here: http://www.bb-elec.com/tech_articles/FAQ_rs232_connections_work.asp

You connect all corresponding signal lines, like for like, name for name, label for label. No muss, no fuss. What could be simpler?
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Offline Psi

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Re: connecting tx/rx to an rs232>usb converter
« Reply #13 on: October 08, 2011, 01:21:51 am »
yeah,  SPI does it the right way.
It's obvious what it means and how you should connect it.
But look at the top two pictures here: http://www.bb-elec.com/tech_articles/FAQ_rs232_connections_work.asp

You connect all corresponding signal lines, like for like, name for name, label for label. No muss, no fuss. What could be simpler?

It might be simpler for someone just wiring a plug, but at some point somewhere TX has to be connected to RX and the point at which this distinction is made is unlikely to be the same over all devices.

It may change over at the pcb silkscreen level, the wiring level or you might get some weird chip manufacture labeling their actual chip pinouts with TX as RX if the IC is a slave only device.


« Last Edit: October 08, 2011, 01:24:25 am by Psi »
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