USB-RS485 Daisy Chaining USB

[fifty60]

I am currently stuck with a PLC that only has RS232 and RS485. I can easily add a USB to 232/485 converter. However, if I cannot use the USB to daisy chain several PLC's.

RS485 simultaneously transmits to all devices on the network. There is no retransmitting the signal. The same signal gets applied to all devices.

Would I be able to take a single USB to to RS485 converter for the main PLC, and then daisy chain the converted RS485 between the PLC's by using standard USB connections? I would use USB cables and connectors but have them connected to RS485. I currently use RJ11 jacks, but wouldn't the USB connectors work the same?

The USB connectors only function would be RS485.

Any other suggestions on how to daisy chain USB's?

 

[GoldDigger]

Using multiple USB to RS-485 converters is a non-starter, since they would not be able to properly control who was transmitting and who was receiving from the RS-485 bus. (I would call the interconnection a bus rather than a daisy chain just to emphasize the electrical setup as opposed to the mechanical configuration.
The computer will be the bus master, I assume, and so it would be best to locate the USB to 485 adapter right at the computer.

If the PLCs have RS-485 connectors (in the form of modular jacks or a D connector) I do not see why you are interested in using USB cables and connectors to extend the bus from PLC to PLC. You cannot use a USB hub to make your multiple connections, so there is no advantage there. Whereas you can easily get modular two to one couplers that would allow the chaining.

 

[Jraef]

I am currently stuck with a PLC that only has RS232 and RS485. I can easily add a USB to 232/485 converter. However, if I cannot use the USB to daisy chain several PLC's.

RS485 simultaneously transmits to all devices on the network. There is no retransmitting the signal. The same signal gets applied to all devices.

Would I be able to take a single USB to to RS485 converter for the main PLC, and then daisy chain the converted RS485 between the PLC's by using standard USB connections? I would use USB cables and connectors but have them connected to RS485. I currently use RJ11 jacks, but wouldn't the USB connectors work the same?

The USB connectors only function would be RS485.

Any other suggestions on how to daisy chain USB's?

You likely don't have enough power in a USB port to connect more than one device.
I don't understand the use of multiple USB connections though. Generally if you have an RS485 connection in the PLCs, you have the ability to daisy chain them to set up a local network using Modbus RTU protocol, so each PLC would have a unique address on that network and only respond to packets addressed to it. That is typically limited to 32 nodes, but do you want more than that? If so, you should be looking at a better PLC, one with Ethernet. What brand of PLC is it?

 

[petersonra]

RS485 simultaneously transmits to all devices on the network. There is no retransmitting the signal. The same signal gets applied to all devices.

That is the way RS485 is supposed to work. If you device supports some kind of multislave protocol it should work fine.

Would I be able to take a single USB to to RS485 converter for the main PLC, and then daisy chain the converted RS485 between the PLC's by using standard USB connections? I would use USB cables and connectors but have them connected to RS485. I currently use RJ11 jacks, but wouldn't the USB connectors work the same?

you cannot daisy chain RS485 by using USB hubs. They are just completely different.

 

[fifty60]

I could use a USB to to RS485 converter on the first PLC, and then daisy chain the remaining PLCs using RS485 over RJ11 connectors. This would basically be the same as using RS485 to begin with, but that initial connection to the first PLC in the line would be made via USB.

The problem is that most people do not have RS485 to begin with on their computers. They then have to use USB to RS485 adapters, and they often have problems. I would like for it to be as plug and play as possible.

So if I get a USB to 485 converter that I know works, then I can install that into the first PLC. The end user then plugs the USB into the first machine, and can daisy chain the rest using 485.

This all works as long as the initial machine can use the USB to 485 converter and not cause any problems down the line.

The PLCs are Idec's Microsmart Pentra line...they do not have any direct USB capabilities...

 

[GoldDigger]

...
The problem is that most people do not have RS485 to begin with on their computers. They then have to use USB to RS485 adapters, and they often have problems. I would like for it to be as plug and play as possible.

So if I get a USB to 485 converter that I know works, then I can install that into the first PLC. The end user then plugs the USB into the first machine, and can daisy chain the rest using 485....

OK, I see your point there. Just be aware that there are some pretty severe length limitations on USB data transmission, so the customer would have to locate the computer within ten feet or so of the first PLC.
See http://www.ebay.com/gds/USB-Cable-Length-Limitations-and-how-to-break-them-/10000000001067998/g.html for more information.
By comparison RS-485 has very long distance capability. (~4000 feet if you have good drivers and a low baud rate.)

 

[Bugman1400]

Can you not daisy chain all the 485s together back to the main computer and then insert your USB convertor there? All devices using 485 protocol will need to have an address (ie 1-255). This address is then used by the software in the main computer to talk to the PLC it wants. The software will ten need to be set up to talk 485 protocol instead of the RS232.

 

[mgookin]

Who is it on here that has the 232 extender company? Is that KZ8? Where's he at?

We use serial all the time for our programmers/ flashing jigs and instruments and use uart usb-serial adapters but I can't say we ever had to connect multiples concurrently and map it all out. For some we use dedicated machines due to the very nature of conflicts. Sorry I can't be more helpful here.