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uC: EM357 datasheet
RS-485 IC: sp3078e datasheet

Two input pins on the sp3078e need to go high at the same time when I'm sending bytes. I'm using a single output pin on the EM357 to drive both of them high and low. The EM357 datasheet says the max output current for the pin that I'm using is 4 milli Amp. The max input current on the sp3078e is 1 micro Amp. Do these connections require any series resistance or pull-up/pull-down?

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Just tie the direction control pins together and drive them directly. I use 3073 with 3.3V GPIO and a TTL UART and it works great. If you are going a long distance - RS-485 on a twisted pair is good for a thousand meters or more - put a termination resistor at the far end. 220 Ohms is good (opinions vary on termination).

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  • \$\begingroup\$ I thought the "standard" was 120 Ohms termination, that's what I've always seen anyway. \$\endgroup\$
    – John U
    Feb 27, 2014 at 11:21
  • \$\begingroup\$ I don't know of any standard. A full termination in some data sheets uses three resistors with a 120 across A and B and a pair of 600 or 620 Ohm from A to V+ and B to ground. I have never used that configuration. \$\endgroup\$
    – C. Towne Springer
    Feb 27, 2014 at 21:18

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