interfacing many device(>250) in single daisy chain. using rs485

hi friends,
i'm using max485 to transmit data serially. and i wanna send and receive data to more than 250 device. can i do with out repeater. i tried this with 3 device, so i didn't put any repeater. help me when you come to know about this.

By specification, no available RS485 transceiver supports > 250 bus nodes. MAX487 is e.g. specified for 120, MAX485 specification is 32, I assume. The basic restriction is imposed by the resistive voltage divider input circuit.

You didn't tell much about the system, e.g intended cable length.

i wanna do more than 1km lenth. and i thought some repeater will help to do this, i mean for this much device. and that voltage divider may avoid reflection in network.

FvM said:

By specification, no available RS485 transceiver supports > 250 bus nodes. MAX487 is e.g. specified for 120, MAX485 specification is 32, I assume. The basic restriction is imposed by the resistive voltage divider input circuit.

You didn't tell much about the system, e.g intended cable length.

Click to expand...

It can be done, but some "gymnastic" with different receivers/transmitters/transceivers is needed, and 4wire RS485 aka Full Duplex, due to load.

For example, your devices utilize the MAX489 as the transceiver, you can then make the "booster" with MAX488 for each differential pair, so that every of the chained devices have only one transceiver load.

So you should CHAIN your devices instead of only dropping them on bus in parallel.

ok. what should i do to do this in half duplex mode.

ZASto said:

It can be done, but some "gymnastic" with different receivers/transmitters/transceivers is needed, and 4wire RS485 aka Full Duplex, due to load.

For example, your devices utilize the MAX489 as the transceiver, you can then make the "booster" with MAX488 for each differential pair, so that every of the chained devices have only one transceiver load.

So you should CHAIN your devices instead of only dropping them on bus in parallel.

Click to expand...

---------- Post added at 17:58 ---------- Previous post was at 17:46 ----------

thanks. what should i do to do this in half duplex mode.

ZASto said:

It can be done, but some "gymnastic" with different receivers/transmitters/transceivers is needed, and 4wire RS485 aka Full Duplex, due to load.

For example, your devices utilize the MAX489 as the transceiver, you can then make the "booster" with MAX488 for each differential pair, so that every of the chained devices have only one transceiver load.

So you should CHAIN your devices instead of only dropping them on bus in parallel.

Click to expand...

Check www.linear.com/docs/4156 , page 18.

I have to correct my previous statement about fanout of available RS-485 transceivers. TI SN65HVD1780 is e.g. specified for 320 nodes. Using transceivers as suggested by ZASto is however a reasonable option.

https://www.ti.com/lit/ds/symlink/sn65hvd1780.pdf

Well I'm actually using SN75179/LTC490 chips (w/o fish :smile for some DMX lighting controllers, connected as boosters and each input side is terminated with 120R. This is to overcome the 32 device limit that MAX485/SN75176 ICs have if the same kind is at the receiving side.
As all my controllers are "dumb" ones, not RDM enabled, this works very well. As each, even commercial controller has DMX In and DMX Out (sometimes called DMX Thru) connectors, to the RS-485 network this daisy chaining is same as parallel drop in.
Of course, the communication is only unidirectional.

I believe, that refreshing the RS-485 signal at each node in a chain of boosters will work reliably in most cases. Strictly spoken, the differential skew specification doesn't guarantee this solution to work with a large number of chained boosters. At worst case, each booster can distort the bit pulse width in the same direction. You have to check the differential skew specification of the involved transceivers. I can just tell, that differential skew is an actual problem when repeating RS-485 data at high MBPS rates.