Yes!
You can easily just add 4 emonTx via usb-serial adaptors direct to the 4 accessible USB ports (more is you introduce one or more usb hubs!)
See the Does the emonTx Shield need an actual Arduino? thread for an example of 3 emonTx’s connected to a Pi via USB’s to create a 3ph monitor. There are several other threads to read as this has come up a few times.
Whilst Bill is correct that only a single channels is actually used, there are 255 different “channels” or “groups” in JeeJib speak, OEM just uses “210”. On any one group/channel you can have up to 30 unique nodes.
Each emonTx comes programmed to use one of 2 node ids dependent on the position of a small switch inside the emonTx, that maybe where the “only 2 channels” originates from. But you can actually edit and reflash the firmware with any valid node id so theoretically you could have 30 emonTx’s on the same channel.
No, not really!
Although theoretically possible, 30emonTx’s all transmitting their payloads on thier own independent “every 10s” cycle with no synchronisation between them means that many (perhaps even most) transmissions will be lost due to RF collisions. Each time the receiver starts to receive a transmission it is blind to any others that might start, in many instances (depending on location) the emonTx is unaware the reciever is busy so it sends anyway, when the receiver finishes the first payload is receives the tail end of the second and discards it as incomplete, whilst it’s doing that it misses the start of the third one etc etc Or the payloads just corrupt each other, either way the data is lost.
It would be totally possible if you were to write your own FW (both receiver and emonTX) to accommodate such traffic, but as it stands even 4 devices unsync’d would be unreliable;e at times. The very first 3ph (3x emonTx’s) I did used rf and the reliability was (IMO) unacceptable, hence the 3x USB emonTx’s in a box approach.
Me too. Even with “reliable” RFM networks, there are occasional hiccups etc, if you have the opportunity to use a direct connection you should (IMO). Plus you get the added advantage of not powering the emonTx via the AC:AC adaptor which can result in better accuracy (especially if not using the RFM).
If you decide to go the USB route there are some pointers I can give you specifically regarding running multiple usb emonTx’s that may not be obvious from the many posts on connecting or configuring a single emonTx this way.
[edit - I’ve edited the thread title to better reflect the question]