I have some questions about interfacing RS 485 mode with my Premium PLCs. I have to interface a Caterpillar engine C.C.M module with my Premium PLC. The Chasis Communication module (C.C.M) is 9-pin female connected, but only three wires are used. In my current project there are 5 engines so there are 5 C.C.M modules. Apart from that, I have to connect 5 power meters, which are also RS-485 mode, to my PLC. Can I connect these 10 devices in daisy chain?

Pls help me...

 

Sandeep,

You certainly can daisy chain all of these 11 devices if you follow the RS485 rules.

1. All of the have to have unique device address lower then 247.
2. All of the have to share the same comms parameters eg (9600 baud, odd parity, 8 bits, 1 stop bits).
3. Both ends of this network have to be terminated with resistors (120 ohm typically)

Send me an email at tom.tran @ industran. com if you need further assistance.

 

Yes, multiple devices can easily be connect in a "daisy chain" manner using RS485. There are some "conditions and limitations" when using RS485... speed, distance, loading. Go to B&B electronics for a good reference on using RS485 (http://www.bb-elec.com). Schneider makes an RS485 "splitter" (LU9GC3) that provides RS485 connection much like an Ethernet Switch.

 

Sandeep,

You should be able to daisy chain 10 devices without too much problem (I think the limit is 32 slaves). I assume the PLC is the master. I recommend using a laptop running a terminal emulation to display the message strings. Although the RS-485 and LT RS-232 signals are different voltage, etc. if you connect Rx and Gnd to - and Gnd (leave the RS-485 + and RS-232 Tx unconnected). You will see the slaves waking up as you figure out the message format. Try just 1 slave at first.

Hope this helps,
Roy

 

RS485 don't have any problem to connect 10 pieces of equipment together; the spec. says 32 devices over 1000 meters. Above that, you need a repeater.

You don't talk about the protocol that you will use. If I assume that it's Modbus and that the Premium will be Master, you need to have a PCMCIA card (TSX SCP114) or a seperate comm. card (TSX SCY xxx) in the Premium rack that wil be configure in Master with Comm. parameters that will satisfy all the equipments on the bus.

Be sure that you polarize and line terminate correctly on the Premium side. See the Premium comm. doc. relative to the SCP or SCY to do that.

For the rest of the bus, you could daisy chain all the other equipments respecting the polarity and line terminate the last one. Be sure that no other equipment polarize the bus.

If you use PL7 to program the Premium, you have the capability to send Modbus request from a Debug screen of the Comm port; this is a neet commissionning tool to validate that the comm. with each equipment works.

GH

 

Rs485 comes in two flavours: 2 wire and 4 wire. 2 Wire is actually 3 wire TX, RX and GNd (common).

All devices need to be RS485; a 4 wire device, can be made into a 2 (3) wire if required.

You did not mention the communications protocol.

RS485 is ONLY the method of physically wiring. What language/protocol is going down the wire?

Premium is normally Unitelway/ASCII, or maybe Modbus.

Most important questions:
What protocol does C.C.M. use?
Is is a master or a slave?

They must be compatible. If they are you can multidrop RS485. Most protocols are Master/Slave. That is, one master device can talk to multiple slaves, this is how Modbus works.

 

RS 485 2 wire does not utilize the ground for a signal common. The signal is between the Rx-/Tx- and the Rx+/Tx+ wires. The pair of wires is used for both transmit and receive and only one is transmitting at any time while the others are listening. RS 482 4 wire uses two pairs, one pair for transmit and one pair for receive.

 

You do need a ground reference though (or isolation) to keep the input pins within their common mode voltage range and absolute limits. There should be no signal current in the ground wire. The fact that stuff works (sometimes) with two wires is that they are both referenced to the power ground and fortunately those grounds are close in potential.

Regards
cww

 

RS485 DOES NEED the ground reference. It is true that the signal is differential between the pair of wires BUT, and this is very important, this will only work if the common mode voltage is within the RS485 specifications. The spec as I recall is from -7 to +12 Volts. Some newer transceivers can go beyond this but it is very important to control the common mode voltage. One way to do this is to use a ground wire. Another way is to optically isolate the circuits. In either case, a ground reference is required.

Jerry Miille