I have almost 30 Current Cost Individual Appliance Monitors (I need to monitor the power consumption of every appliance in my home for my PhD project). Unfortunately, I sometimes see drop-outs on a single channel lasting thousands of seconds, which is simply unacceptable. I see these epic drop-outs even if the IAM is within a meter of its EnviR. So data is being lost somewhere between the IAM transmitting its packet and it being sent via XML from the EnviR. I think the IAMs simply squirt a reading onto the RF carrier every 6 seconds without waiting for a "ping" from the EnviR. There are two possible places where the packets are being lost: 1) the EnviR drops packets or 2) RF collisions
1) EnviR drops packets
If the EnviR is busy processing a packet of RF data when a new packet of RF data arrives then maybe it will fail to receive the new packet. So if two IAMs send packets in quick succession then the second to send will be ignored. The RFM01 only has a 16 bit buffer so it could easily overflow. I have experiemented with setting two EnviRs to receive data from a single IAM. Sometimes both EnviRs receive a packet; sometimes only one will receive a packet and sometimes both will drop the packet. I take this as evidence that sometimes an EnviR will drop a packet because it's too busy.
2) RF collisions
An alternative explanation for the long drop outs is that some of the failed IAM transmissions are caused by RF collisions. How likely are RF collisions? Apparently the Current Cost devices use a 4kbps data rate. So a single bit take 1/4000 of a second to transmit so a single byte takes 8/4000 seconds = 2ms. The RF packets on the CC transmitter are 16 bytes long
. So a single packet takes 16 x 2ms = 32ms. So about 30 packets can fit into a second and 180 can fit into the 6 second gap between IAM transmissions. Let's make the maths simple and assume that we have 180 discrete time slots per 6 second cycle. The chance of a single IAM transmitting in any given time slot is 1/180. If we had only two IAMs then the chance of them sharing a single time slot (and hence colliding) is 1/180 x 1/180 = 1/32400. But we have 30 IAMs hence we have a total of 30-choose-2 pairs = 435 pairs, so the chance of any pair colliding is 435/32400 = 1.3%; which is rather too high for comfort given that I want this logging to run for months and months. And of course there are several reasons to believe the chance of a collision is even higher: we don't have discrete time slots and collisions can happen between any set of transmitters, not just pairs. Ick.
First I'm going to assume that the main problem is that the EnviR drops packets because it's too busy. Hence I want to connect an RF receiver directly to my laptop in order to sniff IAM data directly from the air without having to use an EnviR. I'm somewhat out of my depth here! After a bit of googling, I came across this Nanode IRC conversation about sniffing the SPI bus of a Current Cost to reverse engineer their protocol. I assume I just need a Bus Pirate to sniff the SPI bus of the EnviR to get the initialisation commands the EnviR sends to its on-board RFM01 RF module; and then I can buy an RFM01 module and connect this to the bus pirate's SPI bus to communicate directly with the RFM01 from my laptop.
If I find that RF collisions are a major problem then I may investigate the EDF wireless transmitter plugs. These are similar to the Current Cost IAMs except, crucially, the EDF models use transceivers and not just transmitters. The EDF Eco Manager base station "pings" each transmitter plug in sequence and the transmitter plug responds within about 20ms. This should totally avoid RF collisions. The problem is that I already have 30 Current Cost IAMs! I'm planning to take one apart to see if there's any possibility of converting it to a trasceiver type (the IAMs say "transmitter only" on the back). If not then I guess I'll have to try to return or eBay my IAMs and buy EDF transmitter plugs. I'll still have to build my own transciever because each Eco Manager can only handle 14 transmitter plugs. If I use multiple Eco Managers then RF collisions will become possible again.
Below are some notes on tools and forums...
SPI to USB converters
Forum threads and blog posts
Current Cost specs