What do I mean? What I'm really asking is has anybody out there found a PC or MAC or UNIX machine that will not communicate with a PIC directly? We have always had the option of putting the 20K resistor on an input pin and receiving the data from another "true" RS-232 device and I have yet to find a platform that will not receive the TTL level signals transmitted from the PIC. If the USART receive pin is used, we would need a converter (MAX232 or equiv), or a simple FET to invert the signal.

The reasoning behind this question deals with hanging multiple PIC's on a single RS-232 network. I've been doing just that for 15 years now and have not had any failures or communication errors in my workplace (an industrial environment). I am now going to manufacture a device that will go to market and I'm debating (with myself) whether or not I should place a MAX232 on each device. If I do then I immediately limit myself to the number of devices I can hang on a single serial port because the RX pin on a MAX232 has a 5K input impedance. Without the MAX232 I could easily parallel 20 PICS on one line without any problems. I know what you're thinking... why not put a RS-485 interface instead? Well I did that and I actually end up getting more comm errors than with no converter chip at all. There seems to be an added complexity with RS-485 in which the end termination needs balancing resistors based on the number of devices you have connected. I'm starting to get the impression from trade journals that industry is moving away from 485. The end user of my product might use between one and twenty units on a single serial port and I want the comm link to be stable for all situations.

I welcome your comments and exeriences.

John