Can't get AB Micrologix 1100 to respond as Modbus RTU slave

I have configured a Micrologix 1100 as Modbus RTU slave but it won't respond to any request for holding register data. I have a very simple program running that copies the data from an attached RTD I/O module into file N9 (which is mapped to Modbus address 4XXXX). Using default comm interface and RSLogix software, I can see the PLC program running - the registers are updated with temperature information, but when the unit is returned to the "as programmed" run state, it never responds to Modbus requests. The Modbus configuration is 9600, 8, n, 1, no handshaking, and the node address is 1. I'm using the 1763-NC01 interface cable. What am I missing?

Are you certain that the Modbus RTU Master is using the same serial framing (9600/8/N/1) ?

It's very common for different vendors to use different labels on the RS485 data lines. Rockwell consistently calls them "Data A" and "Data B", but your Modbus Master might call them "Tx" and "Rx". Try switching the data wire polarity at the 1763-NC01 cable. Rockwell also uses an isolated RS485 Common wire, while many "2-wire RS485" interfaces rely on Ground or Shield. Make sure there is a connection between the 1763-NC01 Common pin and whatever your Modbus Master uses as a reference.

Another test would be to use an ordinary 1761-CBL-PM02 serial cable to connect the MicroLogix 1100 to Modbus RTU Master test software like the demo version of ModScan32 from WinTech software.

jconder said:

I have configured a Micrologix 1100 as Modbus RTU slave but it won't respond to any request for holding register data. I have a very simple program running that copies the data from an attached RTD I/O module into file N9 (which is mapped to Modbus address 4XXXX). Using default comm interface and RSLogix software, I can see the PLC program running - the registers are updated with temperature information, but when the unit is returned to the "as programmed" run state, it never responds to Modbus requests. The Modbus configuration is 9600, 8, n, 1, no handshaking, and the node address is 1. I'm using the 1763-NC01 interface cable. What am I missing?

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ur lucky u set up the micrologix 1100 as modbus rtu, im also doing same thing not i have timeout error, can u give me your RTS OFF AND RTS SEND DELAY in msecs or InterChar.Time delay and Pre Transmit delay.

thanks

The timing parameters that somebody else is using are not useful to you unless you know the timing parameters of your Modbus master.

Are you using RS-232 directly to the Channel 0 serial port of the controller, or are you using RS-485 and the 1763-NC01 cable ?

What baud rate, parity setting, data size, and stop bits settings is your master using ?

Until you know for certain the configuration of the Modbus master, you are only guessing about how to configure the Modbus slave.

Eddie Willers said:

The timing parameters that somebody else is using are not useful to you unless you know the timing parameters of your Modbus master.

Are you using RS-232 directly to the Channel 0 serial port of the controller, or are you using RS-485 and the 1763-NC01 cable ?

What baud rate, parity setting, data size, and stop bits settings is your master using ?

Until you know for certain the configuration of the Modbus master, you are only guessing about how to configure the Modbus slave.

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thanks for the reply, im using 1761-CBL-PM02 cable one side to connected to RS232 port of my pc and the other side to the channel 0 of ML1100 controller

==SLAVE=
with MODBUS RTU SLAVE configuration,9600,NONE parity,1 stop bit,8 data bit,
NO HANDSHAKING as control line, interchar.timeout=0ms,pre transmit=0ms,

==MASTER== MODBUS POLLV4.3.4
9600,NONE,1 STOP BIT,8 DATA BIT
same with the slave setting except for the MASTER
timeout setting =3000ms
delay between polls=100ms
no flow control e.g.,RTS,CTS,DSR

tried several timeout setting with Master but TimeOut error persistently occur.

any help and suggesstion most welcomed.

thanks

Pls check whether your comm port used to go online with your PLC is consumed by the RSLinx. If yes remove the RS-232 DF1 devices driver from your RSLinx and try again. This is to ensure that your Modbus simulator software get access to your PC comm port to actually communicate with your PLC.

allentcm said:

Pls check whether your comm port used to go online with your PLC is consumed by the RSLinx. If yes remove the RS-232 DF1 devices driver from your RSLinx and try again. This is to ensure that your Modbus simulator software get access to your PC comm port to actually communicate with your PLC.

Click to expand...

finally i got it, i check with the different Modbus Master and error was display "03-illegal Data paramater/value" meaning the error was not on the hardware side but on programming side, the modbus master polling as much as data, whereares, the slave modbus map was not sufficient data in which the master is requesting, when i complete data data for slave modbus map, e.g. 1 WORD data and configure the master length of words is same to the slave.
then, stream of data exchange between the client and controller was successfull.

i thank the guys in this forum for the idea which helps me go on discover which was i failed. more power:site: