piclist 1998\10\26\194556a >
Thread: protecting the RX and TX serial port lines from external DC voltage?
picon face BY : Ry Lato email (remove spam text)

I am using a PIC16C77 are part of a consumer product. The RX and TX
lines of
the PIC are routed external from this product for serial communications
another PIC based device. The SCI port is configured for asynchronous
operation at either 2400 or 4800 baud (speed decision still needs to be
made). The TX, RX and a GND line are coupled to the other device via a
3-conductor headphone cord with a 1/8" headphone plug on each end. Since
product is for the automotive environment which will be installed by the
consumer, one of the 1/8" headphone jacks may come in contact with
This would apply 12V directly to the RX or TX line causing immediate
destruction to the I/O of the PIC. Although a potentially rare
this type of scenario did happen in one of our prototypes. The headphone
was rested on top of the car battery. Possible solutions I have

I can use 2 10K series resistors between the RX and TX lines to the
outside world. This would limit current to the port in the event of
applied 12VDC. This limited current should be safely dissipated by the
diodes on each pin of the PIC. But the 10K series resistance coupled
with the 10-15ft of cabling will create an RC network that will severely
attenuate/degrade the serial port signal.

I could add a 5.1v zener diode in paralell with the port pin and GND in
series with a 50-ohm resistor. But this is ineffective if the PIC's VDD
drops below 5V for any reason, such as a brownout or if the unit is
powered down.

I could add a .6V silicon diode between the PIC pin and VDD, with a
50-ohm series resitance. But that would dump the external DC voltage
directly into the 5V VDD rail
through the diode.

A polyfuse, which is an auto-reset current limited would not work, since
they don't have values that go into the low mA range, nor are fast
enough to limit current before damage occurs.

I have considered using an IC solution for this. I have seen several
RS232 ICs from
Maxim. But RS232 has many more signals than a simple TX and RX. I guess
what I am looking for is a simple 2-port transceiver buffer with
transient and DC voltage protection, in a through hole package. A device
that has low standby current (<500uA) would be ideal.

A solution that doesn't require adding another IC would be my
preference. So far it looks like my only choice. I would appreciate any

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