SN65HVD485E / Equivalent input and output

Hi Ryuji san,

thanks for your request.

The equivalent input and output schematic diagrams do only model the behaviour of the pins. They do not show the components actually implemented in silicon. I talked to one of our designers why the different transistor types are used in the schematics and this is his answer:

1. CMOS for D, RE, DE inputs and R output: These are CMOS push-pull stages for logic levels, actually implemented similar to this.

2. JFET for A and B outputs: Represent high-voltage capability for the bus pins.

3. Bipolar for A and B input: Representation for low-offset inputs.

Also be aware that the A and B inputs are connected to the bus and thus have all additive noise of the input signal when regarded as single lines. The use of A and B as a differential pair in RS-485 cancels this additive noise on the receiver side.

Your consideration about the logic i/o is correct. Due to their use case they do not need a high voltage rating ( –0.3 V to Vcc +0.3 V) while the bus pins do (-9 V to 14 V).

Best regards,

Christian Klickow