RS-485 arrangements

Full duplex (FDX) RS 485 can be wired as point-to-point (typical use) or as point-to-multipoint (unusual but possible). If you use the point-to-multipoint configuration, you are correct that it looks effectively like the half duplex configuration, because you still need a master and slave(s) relationship. Replies from the various slaves have to be non-overlapping, and their output transceivers need to be controllable just like the 2 wire HDX configuration. So its hard to see any benefit to the extra wire pair in point-to-multipoint.

A point-to-point FDX configuration is usually used when you need high throughput, or when you don't have access to the 485 transmitter enable signal. If you are sending command-reply type messages, FDX doesn't increase throughput because the two devices are alternating their messages anyway. Most FDX 485 configurations with which I am familiar are really FDX hardware with HDX software implementations.

Steve

Basic rule for any kinda bus. Only ONE device can drive the bus at the same time. If two (or more) devices attempt to drive the same bus at the same time it's called a bus fight. The bus may go high or low or halfway inbetween, depending upon the strength of the bus drivers doing the fighting. Does not matter what kinda bus, you cannot allow two bus drivers to drive at the same time. The only common solution to bus sharing is a strict master-slave arrangement. If other solutions exist they ain't common.

David Starr

Full duplex fully, overlapped is totally possible, but all the slaves have to have a 'system address'. A tip, design your system for the master to send, say, x bytes per packet, and all slaves to return, x-1 bytes per packet..

slave devices don't talk unless spoken to. this means, You need a protocol. MODBUS is a good example.. Look it up.

Far too complex...... just use for a packet, address/address/command/parameter/parameter/crc. The biggest task is defining the command/parameter set that will satisfy your application..

Re-inventing the master/slave protocol is far more complex.

There are code examples of MODBUS out on the net that that OP can find and use.

Maybe even use whatever MODBUS code thats out there and modify it to make it simpler for his application.

Code reuse is a good thing.

don