Lee Jones wrote: >>> I presume that it would work because of the low currents involved and the >>> fact that RS-232 "0" can be represented by 5V. Does this mean that a 1 is >>> anything less than 3V (which is what i believe the EIA Spec says). >> >> The very simple, answer is "YES." This is because any non valid input >> voltage is defined as a logic 1 on the output. Don't become confused, >> this is the essential point. BUT, as Mike Fahrion pointed out, there >> are a some cases the negative swing is required. >> >> RS232 input to a pic can be through a single resistor [...] > >If the PIC is receiving data via RS232 from another device, then this is >true. The sender will generate both positive and negative voltages. You >should have a resistor to limit pin current and a diode to ground to keep >the PIC's input pin from being driven to far below 0V. Be aware that you >are giving up some noise margin, but it should work fine. > >If the PIC is trying to send data via RS232 output to another device, it >won't work to significant numbers of devices. It may work fine to a PC >or a Mac, but... Lots of equipment wants to see the negative voltage. > If you need that negative voltage... I slapped together a 16c54 based data logger that would download recorded data to a PC over the RS-232 port. I wanted to be able to to talk to the logger with a standard terminal program to set the current date/time and also to instruct it to do a data dump. I also wanted it small (final design with an eeprom, I2C RTC, and battery was the size of a match box). Data was bi-directional but half duplex. I used the standard current limiting resister trick for the pc to pic link (with a diode) but then I used the pc tx line as a negative voltage source for the pic to pc link (with a resister and a npn transistor). Minor note: anything the pc sends the pic is echoed back through the bias resister. The circuit worked fine, (even with the pic running on a 3volt lithium cell) but as mentioned earlier this is definitely a mickey mouse solution. I would not trust it in production designs. Hope this helps a bit..... carl ---------------------------------------------------------------- Henry Carl Ott N2RVQ | talk/chat carlott@204.74.7.186 carlott@interport.net | http://www.interport.net/~carlott/ ---------------------------------------------------------------- "On justice and on friendship, there is no price, but there are established credit limits."