SN75ALS181: Interface forum

Duplicate of SN75ALS181: RS485 network.

This depends on the characteristics of the cable, and how much jitter you can tolerate. If it worked with the SN75ALS181, then it will also work with the SN65HVD3088E.

Hi Yanagisawa-san,

RS-485 standard allows 32 unit loads on the bus - The SN75ALS181 is a 1 unit load device so up to 32 SN75ALS181 on one bus, but the SN65HVD3088E is 1/8th unit load so you can place up to 256 nodes on one bus with the SN65HVD3088E. 

For the distance versus speed question - as Clemens mentioned as well - its variable depending on cable properties and allowable levels of jitter in system. 

The chart that Clemens has attached is the same ones that we use internally to approximate distance versus speed. The setup the chart refers to is  a 22 AWG cable, 120Ω, unshielded twisted pair. It also assumes the bus is connected correctly.

1) This means that both terminal nodes (start and end node) are terminated with 120 ohm resistors

and

2) all unterminated nodes stub length is minimized.  L <= tr,tf(minimum reported value)/10 * Vp ; Vp is phase velocity of signal through transmission medium - a common value is 78% of c - using that appx. Vp value the max unterminated stub length is about 35mm for the 3088E.  If you have a daisy chained network topology this really shouldn't be a concern but the max unterminated stub length for the 3088E will be about 6x - 9x shorter (210mm - 315mm) compared to SN75ALS181 which is being used . This is because the SN65HVD3088E is a faster device - however both devices are typically considered faster and realistically stub lengths need to be minimized in both situations - but reflections at signal transitions will happen at shorter unterminated bus lengths on the 3088E. 

Ultimately the NSR  variant of the SN65HVD3088E was made exclusively to replace the SN75ALS181NSR - as it turns out a lot of the SN75ALS181NSR applications force the device to be half duplex (which also seems to be the case in your application) so the half-duplex SN65HVD3088E in a compatible footprint to the SN75ALS181NSR can be used as a drop in as long as the SN75ALS181NSR was being used as a half-duplex device to begin with (meaning that the differential driver pair and receiver pairs are shorted together externally). We haven't seen issues on the faster nature of the SN65HVD3088E - but it is something to be aware of when looking to make a switch. 

Please let me know if you have any other questions and I will see what I can do!

Best,

Parker Dodson