Hotplugging UART

[player80]

Hi,

I have a system with around 90 clients those 90 clients should connect via UART to the main system and be hotpluggable.
The cables from the clients (from the controller to the main system is around 20cm).
On the terminal I thought about using optocouplers before passing the signal into an FPGA.
Each of the 90 clients also has his own slot, and the fpga / arm system should detect which slot is occupied (the ARM should query the FPGA via SPI/I2C, I have done SPI/I2C cores earlier already)
The frontend software on the other side can just query the fpga which positions are occupied.
(I thought about a serial protocol first but I need the position information).

Can anyone comment if this is viable/practically?

 

[betwixt]

The design is yours of course and all of the proposed solutions will work but it does seem you are choosing a very cumbersome and complicated way to achieve an end.

You can detect the presence of a 'hot plugged' external UART by simply reading the state of the RX line if you are careful because it will drive a logic level that you can weak pull-up on your side. As regards using opto-couplers, again you can do that but consider the worst case scenario of 90 (or more) of them being driven at say 10mA each needing about 1A of current so you also need some additional driver circuits. I don't know of any FPGA that can supply 1A output current even if it is across several pins.

Brian.

 

[player80]

Hi,

I guess we never had the chance to help you... did we?

Klaus

No you did, it's just in between the lines. No big objection against using optocouplers for the external interface.
I have those few constraints (eg. 2 uart pins per module), plugging them into the optocouplers is cheaper than adding an RS485 transceiver.
I might add an RS485 transceiver however between the FPGA and main processing unit because the cable might be a few meters.
Still thanks a lot for the discussion so far!

 

[barry]

Hi,

I guess we never had the chance to help you... did we?

Klaus

Willful ignorance. Maybe OP should use 180 FPGAs, one for each line.

And I have no idea what they mean by ‘they will output UART only.‘ A UART is a device, not a hardware signal definition.

 

[player80]

you are right I should write 3.3V LVTTL (usually used for uart, certainly there are exceptions too).