| ??? 07/06/06 07:13 Read: times |
#119719 - common mode voltage Responding to: ???'s previous message |
Erik Malund said:
1) common mode limitations: there is no way of predicting how a 485 transciever will behave if a noise spike exceed the common mode limit.[...]
The reason I use shielded twisted wire is #1 above, even with isolated transcievers there is a common mode limit. We always use a pair of (bipolar) transils to protect the transceiver from overvoltage. However, this is a protection, overvoltage still causes malfunction (but not damage). Even with isolated transceivers there is a need to establish a common reference (gnd). With isolated transceivers, I see no reason* why not using an extra conductor - whether it is the shield or an extra wire. There are 8 wires in the common UTP/FTP/STP cable (and, thanks to its usage and mass production, it is sometimes surprisingly cheaper than the comparable cables with less conductors of the same diameter, twist etc.); so after having spent one pair for RS485 (or two pairs for RS422), still there are 4 or 6 left... Jan Waclawek --- * the usual disclaimer: me stupid enough, so there might be reasons... |
