Selecting a Microchip

One affordable solution would seam to be a multiple RS485 network with AVR slave relay nodes being accessed by a PC application. You can have your own unique protocol, or use AvrCo existing MODBUS or SLIP slave protocol implementations. MODBUS RTU is faster then MODBUS ASCII but it is binary so making master pc application is a little harder (Delphi MODBUS libraries exist to help you on this one). Although MODBUS RTU slave would fit in any AVR MEGA with 16KB flash I would recommend some 32KB model to avoid surprises in case of future expansion. SLIP protocol is more modest on flash usage but someone else should be more competent to say more about it. Since we are talking RS485, USB support in AVR is totally irrelevant. AVRs with a single COM port are enough since with half duplex RS485 network, single master and each node having unique ID, only one node will reply at a time. On pc side all you need is one or more USB<>RS485 converters that add RS485 as a COM port to your computer. MOXA makes few models. RS485 supports up to total of 127 nodes (including repeaters) on a cable with up to 1200m in total length. After each 32 nodes you must put a repeater. After each 1200m you also must put a repeater. There can be a maximum of 9 repeaters. These rules force you to use 2 RS485 networks (if you are ready to live with a 2*127-NumberOfRepeaters node limitation) or even 3 RS485 networks if you want full capacity of 255 nodes as in your request. This is easy to achieve since you can use multi wire single cable that holds 2 (or 3) RS485 networks. So in a simpler 2 RS485 networks config you would have a PC with 2 USB<>RS485 convertors, and up to 124 AVR nodes with up to 3 repeaters on each RS485 channel, which means that a maximum of AVR nodes with 2 RS485 networks would be 2*124=248. AVR nodes would need a power supply, termination if they are last node in RS485 network, and a way to have a unique address ID (you can hard code it into each node, or use DIP switches for unique IDs). I would personally make 124 green relay boxes, 124 blue relay boxes, 3 green repeater boxes, 3 blue repeater boxes, 2 USB<>RS485 convertors, and 2 terminator endings. Each box would have 1 input and 1 output multi wire cable, with green boxes connecting to RS485 net number 1 (and just passing through wiring of net number 2), and blue boxes connecting to RS485 net number 2 (and just passing through wiring of net number 1). All AVR boxes would have the same MODBUS RTU firmware, with unique ID read from eeprom and printed on the box. That way users would just need to connect node boxes with appropriate cables, terminate last nodes, connect power, and follow simple rules when to put repeaters. PC application then calls one by one both RS485 network nodes in it's initialization, and enables their visualization and control.

AvrCo MODBUS RTU doesn't work on XMEGA yet, so in that case XMEGA32A3U would still not be an option.

I would agree with Merlin. If you are to have a plug anywhere card and sort out the cards sequence along a cable without setting a physical sequence address on the card, then there has to be a mechanism to find the card ID as you travel up the cable.
You either isolate the comms cable to the next card until this card has logged on to the supervisory PC or you use two ports and do not send messages on until this card is logged on. The advantage with two ports is you get a repeater of types, built in to all cards
This is not new, it's the way most addressable fire alarms work.
Taking a leaf out of fire alarm electronics technology you could use the power cable as the comms cable and modulate a DC supply cable with voltage and current pulses, voltage is usually the master and current is usually the slaves communication channel. However this may be a bit ambitious for our enquirer.
best
Crusty

Hi Avra.

The max distance specified was 50m. I am only talking of peer to peer, not multidrop - (so not many slaves!). Under these circumstances I think CAT-5e will be fine. The only problem I see is if power is to be sent over the cable. If it is locally powered (e.g. batteries) or if power consumption is very low, I see no problems at all. Customers like it (because it is easy to plug in or out and easy to get spares).