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Switching Pull up / down resistors in/out of circuit digitally

Switching Pull up / down resistors in/out of circuit digitally

(OP)
Hello,
I am tying to come up with a circuit to allow me to switch some pull up and pull down resistors in or out of the circuit on some differential signal lines (RS485 fail safe bias resistors). I need to get the enable for the pull ups across a opto to my isolated driver circuitry. Is there a easy way to switch a pull up resistor in/out from a digital signal. Many micros have internally enabled pull ups so they must do it somehow. I tried using MOSFETs and BJTs but havent been able to make that work, I think I am getting base-collector/body diode current flowing when I don't want it to causing excessive current draw and bus loading.
regards,
-Jeremy

RE: Switching Pull up / down resistors in/out of circuit digitally

Use the optos themselves as a way to turn them on and off?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Switching Pull up / down resistors in/out of circuit digitally

(OP)
I am actually using digital isolators not optos because the power consumption is much better and this needs to be as low power device as possible. The digital isolators have logic outputs...

RE: Switching Pull up / down resistors in/out of circuit digitally

Well... Just get a MAX13450E. It's totally bulletproof and has turn on-able pull ups along with wire reversal and a fault output.

MAX13450E

Keith Cress
kcress - http://www.flaminsystems.com

RE: Switching Pull up / down resistors in/out of circuit digitally

(OP)
I looked at the MAX13450 when i was initially designing the prototype, it has switchable termination but not bias. I am thinking of using a CMOS switch such as the
SN74LVC1G3157 but am a little concerned with possible over voltages exceeding the 5V rail of the analog switch (RS485 is spec'd to -7 to +12V) but since the port is isolated it shouldn't have much for common mode voltage (i don't know of any drivers that actually drive more than 5V). I have used an analog switch to switch termination resistors in and out of circuit and it seems to work OK.

RE: Switching Pull up / down resistors in/out of circuit digitally

How about using a fail-safe RS485 transceiver and not bothering with all this switching? TI SN65HVD12D comes to mind.

Z

RE: Switching Pull up / down resistors in/out of circuit digitally

(OP)
I am using a fail-safe transceiver, however since this device is just a repeater/converter meant for existing industrial networks I have no control over the driver architecture of any of the connected devices and deal often with pretty old legacy PLC's that may require a bias but do not contain bias internally. I hacked in some analog switches (SN74LVC1G3157D) in place of the FET's I originally used and it seems to work well, it was a pain to solder leads to the SOT23-6 package, but I got it working enough to do some tests.

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