485 Network With Additional Serial Port

[kerry1150]

We are trying to create a 485 network with:

1 PC running Tri-Excel Link with an Auto 485 converter
2 T100MD+ plc's each with Auto 485 converters for
   additional serial ports

Is this possible to do?  Does anyone know how to node
the Auto 485 converters so that the plc will recognize
the correct one as comm 3?

Also, has anyone tried to add multiple Auto485 converters
to act as serial ports to a single plc?  I have a few projects
that involve a barcode scanner and serial comms with a PC,
and it would be nice to have a programming port available.

Any help would be greatly appreciated.

[support]

You need just one Auto485 converter on the PC and you can connect to as many as 255 T100MD888+ PLCs using their built-in two wire RS485 port. You don't need an Auto485 at each PLC to do this.

You will only connect Auto485 to the PLC if you want to convert their RS232 port to RS485 port. In this case you will need a null modem cable to connect the PLC's female DB9 to the Auto485 DB9. The connection is simply 2-3, 3-2, 5-5.

You don't need to program the Auto485 with a node ID (you can't do it anyway) as the PLC's built-in protocol automatically handles the ID in the host link command or MODBUS command using the user-defined ID on both serial ports.

However, if you actually need two RS485 ports on each PLC, we would recommend that you connect the PC to the built-in RS485 port (comm3) for master slave communcation and use the second RS485 port (converted from RS232 via Auto485) for connection to other RS485 devices such as HMI or I-70xx modules.

[benb2000]

My application requires 3 RS232 serial ports, on a single T100MD-888+ PLC. How is it possible to connect 2 Auto485 units to the one RS485 port on the PLC?

The User guide for the Auto485 has a paragraph heading" Adding a Second or Third RS232 Port to an M series PLC"., but only gives an example for a single Auto485.  

How does the T100MD888+ handle two Auto485 units on the 2 wire network without ID numbers?

Is it possible to use two Auto485 units with one PLC?

Do I just PRINT #3 and PRINT #4 etc.?

Any examples available?

[support]

T100MD+ only has two serial ports. You can convert the RS485 to a second RS232 port using one Auto485.

Are you receiving any asynchronous input string from the second or 3rd RS232 port? If these two serial ports are mainly output port or act as a master port (i.e. the T100MD+ will be the one initiating a command and it may or may not receive a response), then it is possible to use a dual pole dual throw relays to switch the pair of RS485 signal between two Auto485 adapter. The relay is controlled by the PLC's digital output. When the PLC wants to send data out of the second RS232 port it will control its digital output to switch the relay to the second RS232 port and if necessary, wait for a response. When it is done with second RS232 port it can then switch the relay contact to another Auto485 that is connected to a 3rd RS232 device.

However, if the PLC need to receive asynchronously data from the RS232 device then this method would not work. You may need to consider the T100MX+ PLC which has 2 RS232 port and 1 RS485 port.

[benb2000]

Thanks for the info.

[benb2000]

Reviewing the datasheet for the Auto485, is it possible to control two Auto485 units on a single T100MD-888+ COMM3 485 port, by having the PLC control the RTS line of each Auto485?

Can the Auto485's be tristated?

[support]

No quite likely since the RTS need very precise timing and speed that the PLC program is unable to follow.

Also the RTS line on the Auto485 is not accessible by the PLC from the RS485 side. The device on the RS232 could control the RTS line for a more precise turnaround of the RS485 driver, but this would not prevent two RS232 devices from trying to talk at the same time and therefore corrupt the data.

If the two RS232 devices can be controlled by say a DTR line then it may be possible to use the PLC's output to drive the DTR line (not directly, you need electronic interface to shift the level from the PLC's 24V output to the DTR RS232 signal level) of the RS232 devices in order to control their transmission.