Modbus into crio serial port pin layout

[paperduck]

Hello,

I'm trying to connect an instrument over Modbus to a CRIO 9074.  I've gathered from searching online a couple of potential ways of doing this but I'm not sure.

1) One way that seems will work is using a NI9871 module.  This seems pretty straightforward, however I will need to add this module to a 9144 EtherCAT under scan mode, will there be any problems with this?  I will be spending quite a bit of money going this route.

2) Can I connect the Modbus from the instrument directly into serial port on the 9074 chasis??  This appears to be a valid route reading from http://sine.ni.com/nips/cds/view/p/lang/en/nid/201711.     If it is, please tell me which pins on the DB-9 serial connection the T-, T+, R-, R+ from modbus connect to.

DB-9

Pin. Signal

1. DCD

2. RXD

3. TXD

4. DTR

5. GND

6. DSR

7. RTS

8. CTS

9. RI

Thanks in advance,

pd

[Wayne.C]

paperduck,

The serial port on the 9074 is RS-232.  Your pin descriptions sound like your instrument uses a RS-485 port.  If that is the case then a simple ( inexpensive) rs485-rs232 adapter could be installed between the RS-232 port on the 9074 and your instrument.

[paperduck]

Wayne,

Do I then use the visa vi's and parse them like a normal string over serial or is there another step involved in Labview?  Thanks.

pd

[Wayne.C]

I would start with the Modbus_lib for labview.  It contains a set of modbus functions that use NI-VISA to communicate over serial or ethernet.

[paperduck]

Wayne, thanks I've ordered that converter.  The reason why I asked was because the equipment had the option to use RS-232 or RS-422/485 depending on jumper configurations for its output terminal.  I configured it to output 232, but I'm not seeing anything on the RIO side.

[Wayne.C]

Can you post the manual for the instrument?

[Toriki]

Hello,

I am facing a similar problem.

I would like CRio 9004 to communicate with a controller : 

- using the Modbus protocol.

- using the crio's db-9 (de-9), rs-232 serial port.- knowing that my PLC (controller) communicate through rs-485 (2-wires protocol) or rs-422 (4-wires protocol) network.

Although I am starting to understand the "balanced/unbalanced problem thing", 

I am still wondering whether there is a simple way to connect my device to the crio rs-232 port (I mean without using any rs232 to rs422/485 adapter) or not.

I think this won't work but still, can the following work?

connecting both my device's Rx- and Tx- to the db-9 Gnd pin5,

and Rx+ (resp. Tx+) to db-9 RxD pin2 (resp. db-9 TxD pin3).

(Thus without using DTR/DSR or RTS/CTS) ???

If this is feasible, what are the necessary conditions, voltages limits, ... ?

I hope this was clear enough, and thanks in advance for any answer or suggestion

NB : I just need to collect data from the PLC for further treatments (with others measures using crio modules)

NB2: I am French and I apologize if my English isn't good enough

Toriki.

[Wayne.C]

Toriki,

 You need an adapter.  You can't connect a RS-485 port directly to a RS-232 port.  RS-485 uses a differential mode of operation while RS-232 uses a single ended mode of operation.

These adapters can be purchased or you can build your own.

[Toriki]

Thank you Wayne, although I hoped you would have told me that there's a simple way as I might use the rs485 network with maximum 2 slave devices.

Am I right if I think that using a rs485 - 4 wire network will be easier to implement/develop on LabView?

(Since there wouldn't be any need of sending/handling DTR/DSR (or RTS/CTS))

[Wayne.C]

Just remember, you only need one 232-485 adapter.

There can be advantages to a 4-Wire ( Full Duplex) 485 network but most of the time a 2-wire is sufficient and probably easier to implement in LabVIEW.  Typically a 2-wire 485 network has one master and many slaves.  Master only talks to one slave at a time and slaves only respond to requests from the master.  No handshaking is used.