connecting tx/rx to an rs232>usb converter - Page 1
Forum section: electronics / beginners

author: dequo
author_rank: Contributor
timestamp: October 05, 2011, 07:38:00 am
content: I have an angle sensing instrument that communicates with rs232 protocol to a com port on the computer viahttp://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, TXthe 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 groundpower connected to power supply groundi 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?

author: IanB
author_rank: Super Contributor
timestamp: October 05, 2011, 07:50:41 am
content: Quote from: dequo on October 05, 2011, 07:38:00 amIt'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.

author: Bored@Work
author_rank: Super Contributor
timestamp: October 05, 2011, 07:59:36 am
content: 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.

author: desowin
author_rank: Contributor
timestamp: October 05, 2011, 08:10:52 am
content: Quote from: dequo on October 05, 2011, 07:38:00 amWhat 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.Quote from: dequo on October 05, 2011, 07:38:00 amWell, if target device operates on TTL levels, then it's likely that you are close to (or already did it) frying it.

author: dequo
author_rank: Contributor
timestamp: October 05, 2011, 08:15:00 am
content: ok, either its labelled wrong or im an idiot. connecting what is apparently TX on one to the TX of the other works...

author: Psi
author_rank: Super Contributor
timestamp: October 05, 2011, 08:17:28 am
content: Quote from: dequo on October 05, 2011, 08:15:00 amheh it's labeled wrong,

author: IanB
author_rank: Super Contributor
timestamp: October 05, 2011, 10:22:20 am
content: Quote from: Psi on October 05, 2011, 08:17:28 amWhy 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.

author: Psi
author_rank: Super Contributor
timestamp: October 05, 2011, 12:24:52 pm
content: Quote from: IanB on October 05, 2011, 10:22:20 amDoesn'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?

author: Chet T16
author_rank: Frequent Contributor
timestamp: October 05, 2011, 12:50:50 pm
content: Quote from: Psi on October 05, 2011, 12:24:52 pm

author: metalphreak
author_rank: Frequent Contributor
timestamp: October 06, 2011, 12:23:06 am
content: The silkscreen/label can have two meanings"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.

author: Hypernova
author_rank: Unknown
timestamp: October 07, 2011, 02:50:15 pm
content: One reason why SPI use MISO/MOSI (Master IN Slave OUT)

author: Psi
author_rank: Super Contributor
timestamp: October 07, 2011, 11:52:34 pm
content: Quote from: Hypernova on October 07, 2011, 02:50:15 pmyeah, SPI does it the right way.It's obvious what it means and how you should connect it.

author: IanB
author_rank: Super Contributor
timestamp: October 08, 2011, 12:03:38 am
content: Quote from: Psi on October 07, 2011, 11:52:34 pmBut look at the top two pictures here:http://www.bb-elec.com/tech_articles/FAQ_rs232_connections_work.aspYou connect all corresponding signal lines, like for like, name for name, label for label. No muss, no fuss. What could be simpler?

author: Psi
author_rank: Super Contributor
timestamp: October 08, 2011, 01:21:51 am
content: Quote from: IanB on October 08, 2011, 12:03:38 amIt 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.