An EIA-485 ("RS" refers to "Recommended Standard", and is an obsolete name) interface is rare on a PC, you'll need a separate interface board for that. The reason is that EIA-232 and EIA-485 are not hardware compatible, despite using the same connector type: EIA-232 uses single-ended signals, meaning that RxD and TxD are referenced to ground. EIA-485 uses balanced signals, meaning that both sent and received data have a positive and negative signal which are each other's negative.
Update
As I originally predicted, the wiring is proprietary and the flow control signals are a mess. @SofaKng has reversed engineered the official cable and produced this table (reproduced here):
RJ45 pin, DB9 pin (female)
----------------------------
1 1
2 6 + 8
3 2
4 5
5 5
6 3
7 4
8 7
Original
You missed a key detail. Their RJ-45 patch cable is null modem wired. That means it is reversed. You got all of your pairs backwards (you connected TX to TX, RX to RX, etc...)
Assuming you are using a straight-through patch cable...
- as you indicated that you are with your comment "standard TIA-568B"
- you must use TIA-568B ordering on both ends of your patch cable
...it goes this way:
Device Signal, RJ-45 Pin#, DB9 Pin#, PC Signal
----------------------------------------------
> DTR 1 --- 6 DSR
> GND 2 --- 5 GND
> RTS 3 --- 8 CTS
> TxD 4 --- 2 RXD
> RxD 5 --- 3 TXD
> DSR 6 --- 4 DTR
> GND 7 --- 5 GND <-- note: repeats, bussed to device #2
> CTS 8 --- 7 RTS
> RI 9 --- 9 RI <-- doesn't actually fit in an RJ-45 (only 8 signals), probably safe to ignore if I read their diagram correctly
The DB9 on a PC is pinned out this way (see figure). Note how it is null-modem reversed from the table listing you provided. That's because their RJ-45 cable is null-modem reversed to cancel it out. Tricky and silly, but that's how they chose to implement it.
Some other thoughts...
Actually, I noticed from tracing in the diagram that they do some very strange stuff with the flow-control signals. For example, they short CTS and DSR on the PC side, but not on the other side. They route RTS on the PC side to DSR on the device side. And other weirdness.
This may be bad documentation, but I suspect they have implemented custom firmware/software that makes use of the flow-control signals in non-standard ways as a means of ensuring that you only buy and use their cables and adapters.
I would suggest that you make two half cables. On one end go RJ-45 to unterminated wire and the other go DB9F to unterminated wire. Then you can twist your way through all of these weird configurations until you get it right. I would start with my suggested mapping. If that doesn't work, report back and I'll give you my mapping for all of their weirdness in the flow-control lines.
Good luck! =)
Best Answer
RS-232 transceivers such as MAX232 are guaranteed to survive connection (inputs or outputs) to +/-25V without damage. That's what they are designed for.
If you're using CMOS levels direct from the MCU chip, the limits are more like Vss - 0.3 to Vdd + 0.3V. You may be able to protect them by adding some series resistance (a few K) and operating at a relatively low baud rate to compensate for the resulting more sluggish waveforms. The added resistance limits the current, but does not clamp the voltage (except via the on-chip protection networks, so it will exceed the voltages but current is limited), which may be acceptable. You could add unidirectional TVS diodes to the pins and a bit of series resistance for more protection.