MAX33049E for applications that require a larger network

Question

I recently learned about an RS-4222 transceiver chip MAX33049E on the ADI website, and the data manual states" They feature a 1/8-unit load, which allows up to 256 transceivers on a single bus"。 I want to use this chip in scenarios where one node sends and receives multiple nodes (over 100 nodes), and the bus is a backplane PCB routing instead of twisted pair. But the cost of conducting this experiment is too high. Can you provide a detailed test report to support the MAX33049E feature? Or you can also provide me with application suggestions, thank you.

ShastaThomas · Accepted Answer

Hello, 
 Thank you for your question. Unfortunately, I do not have a detailed report that I can share with you for a 256-node network using the MAX33049. 
 Let me first note that the 256-node maximum is based on the unit load (UL) calculation in the RS-485 standard. One unit load is defined as 12kohm, and a multi-node network allows up to 32ULs in RS-485. Using the receiver input impedance of 96kohms (min) in the MAX33049E, calculations show that 8 MAX33049E transceivers equate to 1UL, so a maximum of 256 transceivers could be placed on a single multi-node network. Other factors, of course, may limit the number of transceivers on a single bus for a given operating voltage, data rate, or bus length. 
 RS-422, however, is defined as a simplex multidrop communication and allows up to 10 receivers on a single bus, only. If you need to be RS-422 compliant, then your circuit may be limited by the standard. 
 Our recommendation is to build a smaller module - limited to a much smaller number of nodes, like 10-15 - and then correlate this smaller system behavior with your simulation environment. Once correlated, you should be able to scale to a larger network. 
 Thanks, 
 Shasta)

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MAX33049E for applications that require a larger network