> PROBLEM BACKGROUND:
> ===================
>
> I'm using a MAX232 RS232 <--> TTL converter to connect a PC,
>  a PIC, and an 8051.
> Communication paths are as follows:
>
>                                     PC <--> 8051 <--> PIC
>
> The 8051 I'm using has 1 UART which is used for both communication
>  paths (ie I could implement the serial communications
>  in software for one communication path and use the UART for the
>  other, and this would alleviate my problem).
>
>
> PC /  MAX232                     8051                       PIC
>  TX     RX                     RX    TX                  TX     RX
> ____________                 _____________              ___________
>   |     |                       |     |                   |     |
>   |     |                       |     |                   |     |
>   |     ------------------------|-----*--------------------------
>
>   |                             |                         |
>   ------------------------------*--------------------------
>
>
> PROBLEM:
> ========
> The MAX232 does not have open collector o/p's (Please correct me if
>  I'm wrong but I checked the MAXIM '95 New Releases Data Book and saw
>  no reference to same). Therefore, the PIC can't transmit to the 8051
>  because the MAX232 is forcing the line high when not transmitting.
>
>
> POSSIBLE SOLUTION 1?:
> =====================
> To overcome this problem, I came up with the following:
>
>                            +5v
>                            ___
>                             |
>                            | |
>       |\                   | |
>       | \          |/|      |
>   ----|  O---------| |----------------- TxOut
>       | /          |\|
>       |/
>     Max232       Diode   Pullup
>    inverter              Resistor
>
>
> When MAX232 o/p is low,  Diode conducts => TxOut is at 0.7V = Logic 0
>             o/p is high, Diode doesn't conduct => TxOut is pulled up
>                            to 5V.
> I'd like to here comments / criticisms on this work-around or others
>  like it. I tested it at 9600 Baud and it seems to work fine.
>
Well, TxOut(LOW) is the diode drop plus the VOL of the MAX232.  Worst
case, you've probably blown your noise margin.  If this is just
a onesy-twosy circuit that isn't intended for production, you can
usually get away with it.  I wouldn't use this kind of kludge in a
production board, 'tho.


> POSSIBLE SOLUTION 2?:
> =====================
> There is a MAX232 version which does the following:
> "Driver outputs turn off and enter a high-impedance state....
>  when in shutdown mode, in three-state mode, or when device
>  power is removed." Receivers remain active in shut down mode.
> Problem is, I want to have one driver o/p permanently active
>  so shutting down the chip to facilitate communication is not
>  feasible.
>
Well, depending on how the MAX232 three-state mode works, I'd
consider that the best possibility.  Why do you care if all drivers
are off?  As long as you have a pull-up or -down (depending on how
you want your inactive line to float), you should be fine.

Alternatively, you could rig your own OC output using a cheap
switching transistor (2N2222 equiv), a base resistor, and a pull-up
resistor.  Depending on how the numbers work out, you might be able to
even ditch the base resistor.


----
Andy Hill
ahill@boi.hp.com