Barcode Scanner

[fifty60]

I have an RS232 barcode scanner that I am trying to connect to a Class II 5V power supply.

I want to connect everything to the DB-9 of the barcode scanner. So I would have my RX, TX, Sig Gnd, and 5V source all connected to the pins on the DB-9.

I usually leave the "-" terminal of power supply floating, so my signal ground is always floating as well. I know I should use pins 2,3, and 5 for TX, RX, and SigGND, and I should use pin 9 to connect the +5V to. I am not sure where I should connect the "-" terminal from the power supply.

Should I connect +5V to pin 9, and then connect the "-" terminal to pin 5? Doesn't seem right. Would this pin be defined by the RS232?

 

[luckylerado]

Pin 5 should be - for a typical serial DB9

 

[GoldDigger]

Pin 5 should be - for a typical serial DB9

One problem you are facing is that the RS-232 standard does not include five volt power pins in the first place.
The logic/signal levels can vary from +-5 to +-15 volts with respect to ground, but are supposed to be current limited.
The RS-232 standard originally specified a DB-25 connector, so some pins were already sacrificed to get it down to only nine pins.
Is there a standard or convention for power over the same DB-9?
I do not know...

 

[fifty60]

Thats a question I have about this as well. How can RS-232 be powered from a 5V DC source?

 

[GoldDigger]

Thats a question I have about this as well. How can RS-232 be powered from a 5V DC source?

Technically, it can't be. At the minimum you would need both + and - 5V with respect to the signal ground. Some RS-232 receivers violate the spec by taking 0V as equivalent to -12V.
However, it also turns out that some of the RS-232 interface chips designed for use with embedded circuitry contain one or two charge pumps to derive the needed + and - voltages from a single ended 5V supply.
It is also possible to do this in a way that leaves the signal ground isolated from the Vcc ground.

 

[gar]

150617-0949 EDT

fifty60:

In many ways your question is not clear. But GoldDigger has given you some useful information.

See "RS232 Pinout" "DE-9" at http://www.interfacebus.com/RS232_Pinout.html .
This is the pinout created by IBM for the personal computer when they switched from the 25 pin connector to the 9 pin layout. There is no place in this connector for power.

1. How is the pinout defined for your barcode device by the barcode reader manufacturer?
2. Is the barcode circuitry isolated from the barcode reader chassis?
3. Presently how is power supplied to the barcode reader?
4. Is there any other external power than 5 V required by the barcode reader?
5. How much current at what voltages does the reader require?
6. How long is the RS232 cable from the reader to its destination?
7. What is the baud rate?

If your barcode reader uses pin 9 for +5 V power, then most likely pin 5 is the 5 V return (what I would call common and the - terminal of the 5 V supply).

For reliable RS232 communication it is important to not introduce noise on the RS232 common line (pin 5 on the 9 pin connector).

My I232 isolator system provides electrical isolation between the two ends of an RS232 channel, and data transfer up to 4000 ft at 115 kbaud. The I232 is a means to eliminate the ground path noise error problem, and prevent equipment damage. I have done 8000 ft at 9.6 kbaud. As a test I have applied a 1000 V 60 Hz sine wave to a wire in the interconnect cable with no damage to any components and no data transmission errors while the 1000 V voltage was present. With the interconnect cable at about 6 feet from a heliarc welding cable I start to get data errors. This is an unshielded cable using twisted pairs.

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