ISO3080 questions about Isc, Icc2, RL, Rid, Rx function

Hi Asaka-san,

Please see my answers below:

(1) reason of differential input resisstance 54ohm(min), 60ohm(typ) (Data sheet P.2) I think that RS485 termination resister is 120ohm. Is this combination resistance of Rx and Tx terminater ?

The 60Ω comes from two 120Ω resistors in parallel. The 54Ω comes from the RS485 standard. It states in the differential output voltage section to test with two 27Ω resistors in series. So we specify the VOD for this condition as well.

 (2)Icc2 with RL(Data sheet P.2) I can find the Icc2 bus-side supply current with no load only. Could you please let me know the Icc2 with RL ?

On page 3 of the datasheet we state there is typically a VOD of 2.3V into 54Ω. This corresponds to 42.5 mA of current. I would add the 42.5mA with the ICC no load that is listed in the datasheet.

 (3)Short circuit output current(Datasheet P.3) I think that Isc of RS485 specification is 250mA. However, ISO3080 Isc is +/-200mA. Could you please let me know this reason ?

The RS485 standard states the following: “…the peak current magnitude shall not exceed 250mA. This criterion is not to be interpreted as a requirement that a driver be capable of sourcing 250mA.” Therefore, this device has tighter current limiting control of the driver. Under all no-fault conditions, 200mA is more than enough current to operate on the bus.

 (4) reason of "Up to 256 Nodes on a Bus"(Datasheet P.1 and P.4) Could you please let me know the reason of 256 nodes ? According to RS485 specification, 12kohm with 32 nodes on a bus. And ISO3080 input resitance is 48kohm. Thus , I think that bus nodes may be 128 nodes.

The unit load in the RS485 standard states that I driver must be able to drive 32 unit loads with at least 1.5 volts differential output. A unit load can be modeled as a worst case 12kΩ differential load on the bus. Therefore, if we are 1/8 that loading, we can have 8 times the amount of devices. 32*8 = 256

In addition, please take a look at the following wiki-note for more information on the unit load:

http://e2e.ti.com/support/interface/industrial_interface/w/industrial_interface/2575.the-unit-load-in-rs485.aspx

(5)Receiver function(Datasheet P.9  Table 2) Could you please Receiver function with Vcc1=PD, Vcc2=PD?

 The R output pin with be High impedance (hi-Z).

Please let me know if you have any other questions,

John