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
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
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switching Pull up / down resistors in/out of circuit digitally
RE: Switching Pull up / down resistors in/out of circuit digitally
MAX13450E
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switching Pull up / down resistors in/out of circuit digitally
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
Z
RE: Switching Pull up / down resistors in/out of circuit digitally