ADM2587E connects 32 nodes


I'm a DFAE in Japan. I'm received a question by the customer. The ADM2587E says it can connect up to 256pcs transceivers. In this case, the input impedance of the receiver is 96kΩ. In other words, isn't the ADM2587E equivalent to connecting a 32pcs 12kΩ receiver? I understand that this is common to all transceivers because it is determined by the ANSI / TIA / EIA-485-A-98 standard. However, there was no description in the data sheet that 32pcs can be connected, so we are checking it just in case.



  • +1
    •  Analog Employees 
    on Oct 22, 2020 2:01 PM 1 month ago

    Hi Hiroyuki, 

    Your understanding is correct. The  ANSI / TIA / EIA-485 standard allows for up to 32x "1 unit load" devices to be connected to a single network. A "1 unit load" receiver is approximated as a 12kΩ input impedance. 

    RS-485 receivers can present less than 1 unit load to the network. In the case of the ADM2587E, this is a 1/8th unit load device, with an input resistance of  96kΩ. Connecting 8x ADM2587E receivers to a network is equivalent to connecting a single "unit load" 12kΩ receiver to the network. A total of 256x ADM2587E receivers can be connected to the network. 

    If you connect 32x ADM2587E devices to a network, this equates to 32 x (1/8th) = 4 unit loads. This will function correctly, but you could also add an additional 28 unit loads to the network.