Well, the RS-232 vs RS-422/485 main difference on a physical interface level is that RS-232 uses a single
wire for Tx and a single
wire for Rx, while the other two use pairs of wires.
In RS-232 Both Tx and Rx are referenced to a common ground. In other words: RS-232 is voltage driven
RS-422/485 use a pair of wires wires for Tx
(Tx+/Tx-) and a pair of wires for Rx
(Rx+/Rx-). These pairs create in effect a current
loop (actually two loops). In other words: RS-422/485 are current driven
Voltage driven interfaces are cheaper (less wires) but more sensitive to noise
, while current loops show high immunity to electro-magnetic interference
and are preferred in industrial environments (well, *were* preferred, nowadays most new designs are optical fibre).
However, both RS-232 and RS-422 are capable of speeds well into Megabit/s range although it is rare to see them used at such speeds.
Now, with technology cleared up a bit, a little troubleshooting advice from a long term SysOp. The easy way to see if a problem is with the converter or with a device not sending data is:
- disconnect the device from the converter,
- connect pin 2 & 3 together at the converter (assuming a RS-232 DB-9 connector; iow: short Tx with Rx)
- run a terminal emulation program on the computer where the converter is attached and connect with the program to the (emulated) serial
interface of the converter,
- set data transmission
to NO FLOW CONTROL (i.e. no XON/XOFF, no RTS/CTS), any speed and parity/stop bits should do here,
- check that 'local echo' option of your terminal emulator is OFF,
- start typing.
If you see your typing echoed back to the terminal all is well, your converter is transmitting and receiving and its drivers are performing ok.
If doubled echo is seen it is also OK, it means that 'local echo' option was left on..
If no echo is seen, investigate on the PC side.
Now, after checking your converter, remove the short between pins 2&3 and connect data cable from your device to the converter. Set data speed and parity to correct values. Look at your terminal program again. Any incoming data present? If yes, device is transmitting and link is working correctly. Time to configure nav software
. If not, investigate cable and device further.
Hope it helps someone.