RS-232 Buffer circuit

If I understand correct, here is your circuit:

RS232 in -->TTL out-->[Sniffer circuit]TTL in-->Rs232 out

in other words:

[device1] <--> MAX232 <--> [sniffer circruit] <--> MAX232 <--> [device2]

That looks good to me. The fact that each side of the circuit is talking directly with a MAX232 should give you plenty of "isolation". You may need to cross the lines (or just use a null modem connection on one side) if you're talking CPE-CPE or computer to computer.

Other than that I don't see any issues.

fwiw I've had success with the minimalist two-diode type circuit that you linked to. The only difference being that I used the TX pin of the 'sniffer' device as a handy source of -12V to use through a 47K pull-down to make sure the sniffer's own RX didn't float when neither of the snooped-upon devices was transmitting.

Unless you're driving tens of feet of wire, or are running at faster rates like 115.2kbps, the diode & resistor thing shouldn't affect the circuit too much.

If you really want to buffer the signals to TTL and back, there are of course the MAX chips, and even simple old line-driver/line-receiver type chips like the 1488 quad driver and 1489 quad receiver that would do the job.

Hmm, interesting circuit. You might want to use an actual MAX232 as a buffer between the lines you're reading and your two-diode setup. It has an input resistance of 5Kohms which means assuming that the receivers have the same you'll see 2.5KOhms of resistance on the bus. Perhaps a voltage follower with a higher input resistance might be in order - it is possible to break serial ports by trying foolish things (I've done it). Try testing on something that you don't mind destroying first.