Serial help please!

Hello all. I have been assigned the task of getting ready for a new serial device that will be installed in our plant. It is a checkweigher that will send pack weights to my SLC5/05 over RS-232 ASCII. I currently have an older unit connected and doing this job with RS-422. It is connected with only two wires to my CH 0 on my PLC. I don't believe there is any communication from my PLC to the serial device, only serial device to the PLC. When it comes to CH. 0 user settings they all match the default settings of the new device coming. I have a couple questions about this. 1.On my new device I have the option to enable flow control. What is flow control? should I be using this parameter? 2.Based on my attached PLC file are the settings in the project configured correctly to recieve the data from my new device and put it in S10:0 as a string and transfered to N13:19 by a ACI instruction as a readable numerical weight. The data format from the new device is ASCII-6 digit code. 3.Is it acceptable to only use two wires if I do not wish to send data from my PLC to the serial device. Wiring is from (TXD)of the serial device to (RXD) of the PLC and ground straight through.
Basicly I am wondering if this will be as simple as wiring it in and transmitting data.(Nothing is ever so simple)
These may be begginer style questions but I have not done much with serial comms as it is a little before my time. Any input will be appreciated. Also if anybody knows of a good tutorial that describes setting up serial comms it would be beneficial.

The 2 wire connection is called RS485. This is done on a RS422/RS485 port. Communications is both ways. Do not use flow control. 3 wire connections (TX, RX, Com) for RS232 is fine.
If wire length is more than 50 feet, stay with RS422/485. Get a RS232-RS485 converter from B & B Electronics, Black Box, or others.
Read more here.

The SLC-5/05 controller has an RS-232 port only, so if the connection is physically wired directly to the Channel 0 serial port, it must be RS-232, not RS-422.

That's consistent with what you describe about having only a Rx pin and a Data Common pin. Having three wires (Rx, Tx, Common) is the ordinary way to connect two RS-232 devices, but there's nothing that absolutely requires that you be able to transmit to the checkweigher. RS-232 can be one-way with just two conductors.

Handshaking and Flow Control are similar ways of having the two devices on the serial link tell one another it's OK or not OK to transmit. These functions can be done with hardware (Pins 7 and 8, or pins 4 and 6) or with special Start/Stop bytes.

Since this is a one-way ASCII protocol, it probably doesn't need flow control. Leave it disabled.

B&B Electronics is a great resource for plain and clear diagrams of serial connections. They are my usual source for isolators and converters.

Another excellent tutorial website is: http://www.lammertbies.nl/comm/info/RS-232_specs.html

So if I am to leave flow control disabled would I do this as decribed in my new serial device manual: If the external device does not control the hardware flow, communication is possible by short-circuiting the CS and RS signals: The manual also has a diagram of RS and CS shorted on both the checkweigher and the PLC RS-232 connector. I don't believe that this is the case in the current set up. I don't see any terminals shorted on either the serial device or the PLC.


PS: Thanks for the Links to lammertbies, very helpfull.

Yes. Jumper RS (Ready to Send) and CS (Clear to Send) together.

Back in the old days, we had slow printers. The slowest was Teletype at 110 Baud. The data could travel faster than the printer could print. The sender would tell the print "Ready to Send". If the printer was idle, it would answer back with "Clear to Send". When the buffer (memory) filled up, it would drop the "Clear to Send" signal.
With today's electronics, handshaking isn't needed. Some engineers still like to use it, mostly as a status signal. With handshaking, it's easy to detect a disconnected cable, for example.

Great. Thanks for the help Keithkyll and Ken Roach.