Current in RS232 loop

NetNathan

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Nov 2011
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I have been told that I need a resistor across an RS232 connection for it to work.
That the RS232 loop is a current loop and it is 4-20ma.

Is this true?
 
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There are serial current loops. They are not technically RS-232, but carry the same data in the same format so the link can be erroneously called Rs-232.

I didn't recall that the serial current loop device used current with a live zero at 4mA, but it might. I can't imagine that putting to get to RS-232 logic voltages at what, 12V?, you'd need a 600 ohgm resistor.

A relatively low resistance of 600 ohms would 'load' an RS-232 output, but not likely damage the driver.

B&B app note on serial current loops:
http://www.bb-elec.com/Learning-Cen...op/Current-Loop-Application-Note/curentlp.pdf
 
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RS-232 is voltage driven. This may be confusion with RS422 and RS485 data communications. They often require a terminating resistor at the end devices to eliminate echoes.
 
Hello,

>So I can measure voltage across the RX/TX to signal ground to verify circuit?

No, for RS-232 the RX and TX are always referenced to ground.
 
I think the OP has some conflicting information.

This from my experience:
RS232, voltage differentual Tx-Gnd, Rx-Gnd. Have not seen termination resistors.
RS422, voltage differentual. Tx A/B. Rx A/B. Mostly uses termination resistors.
RS485, voltage differentual. Tx/Rx A/B. Always uses termination resistors.
TTY aka 20 mA current loop. Does not use termination resistors. Does sometimes use overvoltage zener barriers.

So what is it. RS232, RS422, RS485 or TTY ?
 
an OLD TTY was current driven with, still used inolder machines you can see the signal with a RS232 indeed with a resistor, of about 500 ohms.
when using 250 ohms it will give you a nice 5 volt signal or 1volt, so you can direct (or with optocoupler) put it in a serial port.
 
I got the RS232 communications problem solved.
It is between an Inficon Vacuum Gauge (BCG450) and an AB7007 Anybus Ethernet/IP converter.
(to convert RS232 signal to Ethernet/IP to send vacuum level to Controlllogix).

I actually had to NOT swap RX/TX between the device and the Anybus module....they had to be wired "unswapped", or RX to RX and TX to TX.

The Anybus module now receives the RS232 data okay. It is a 9 byte string with pressure in Byte 4 and 5 (high byte and low byte). I do not even have to request data, the gauge blasts it out every 20ms.

Next issue is I am not getting data into RSLogix 5000 "ReadTag" and "ReadData" tags.
The tags are configured correctly as to the attached sheet. Which I cannot upload because it is too big, name of file is 1"10-7097-ControlLogix5000 MSG instruction_2.0.pdf"....zipped version is 1.4mb...

When I monitor the tags I see only zeros.


When I monitor the device in RSlogix 5000, I get this error:
(Code 16#109) Connection Request error. Invalid connection size.
Tag: BCG450_IP15:1


Any Ideas????
 
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