How Does One Connect RS232?
RS232 cables use three conductors: a common wire s a ground-return and two communication wires. The communication wires carry data from the output of each piece of equipment to the input of the other piece of equipment.
With RS232 connections, the trick isn't so much landing wires; the trick is landing them knowing that they are connected correctly, making troubleshooting the rest of the system is easier.
By Douglas Krantz
So two pieces of equipment can send data to each other, RS232 carries the data. When connecting the equipment using RS232, there's guessing the RS232 is wired correctly, and then there's knowing that the RS232 is wired correctly.
When troubleshooting equipment, just making the RS232 connection no time to start guessing whether or not the RS232 is wired correctly.
How Does One Know?
When troubleshooting, have you ever wished that the RS232 communication wasn't the issue? I mean when you're trying to find out if a piece of equipment is working, how do you know if it's merely a problem with your RS232 connections or a real problem with the equipment you're working on?
Knowing a little about how the RS232 works is helpful.
RS232 is a two direction data transmission system, and there are only three wires. One wire transmits data one direction; another wire transmits data the other direction. A third wire is used as the ground/return for both data transmissions.
Both of the data carrying wires will normally be a negative 5 to 12 volts as compared to the ground/return wire. (The exact voltage isn't important with RS232.)
Equipment to be Connected Together using Data along the RS232 Wires
Each of the two pieces of equipment has a transmitter connection (TX) to transmit data to the other piece of equipment. Each of the two pieces of equipment also has a receiver connection (RX) to receive data from other piece of equipment. Each of the two pieces of equipment has a common or ground/return connection which is used for both the transmitter and receiver.
The RS232 Transmitter (TX) Supplies the Voltage
The transmitter (TX) on each piece of equipment supplies negative voltage (minus 5 to 12 volts normally), which carries the data. The receiver (RX) on each piece of equipment has no voltage when nothing is connected to it.
Connecting the Common
Compared to any other wire, the common wire or terminal is always going to be positive. Even if it's not marked as common, if it's positive compared to any other wire or terminal, it is the common.
The commons on both pieces of equipment should be connected together.
Connect TX to RX
Now, for both directions of data transmission, connect the wire or terminal with the negative voltage to the other piece of equipment's connection without voltage. That's connecting the TX (transmitter) to RX (receiver).
Confirm the Wiring
To confirm the RS232 is correct, on one piece of equipment, check voltage (compared to the common terminal) on both the RX and TX terminals. Compared to the common terminal, there should be voltage (-5 to -11 volts) on both.
That's actually voltage supplied by both transmitters (TX) on each of the 2 pieces of equipment.
Don't worry about exact voltage, if the voltage is somewhere between 5 and 12 volts, it's fine. If not, if there is voltage on one terminal but not the other (TX and RX), reverse the wires to the TX and RX connections on one piece of equipment and check again.
Try It, You'll Like It
Using this technique, one doesn't have to know details about which one transmits and which one receives. Just make sure that there is negative voltage on both RX and TX (compared to the common terminal) and the RX232 is wired correctly.
When confirmed this way, the RS232 isn't the issue and troubleshooting the rest of the system becomes easier.
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms
. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.
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