diff --git a/README.md b/README.md
index 1773ebe..933566b 100644
--- a/README.md
+++ b/README.md
@@ -84,6 +84,27 @@ Connections 3xPZEM-004T <-> board:
 
 Note: when connecting multiple meters one the same UART bus only keep one of the pull-up resistors (across all meters) on the TX pin (on the opto-coupler output), else the low level is not low enough for the micro-controller to correctly decode the signal.
 
+SDM120
+------
+
+This 1-phase 2-wire electricity meter provides an RS-485 interface using the ModBus RTU protocol.
+For the 3-phase 4-wire power distribution installation I used 3 meters, one per phase.
+
+They can be connected to the same RS-485 bus once individual addresses have been set.
+The used meter IDs (aka. slave address) are hard coded in `main.c`
+A UART to RS-485 converter is used to be able to communicate with the meters.
+
+The SDM120 ModBus protocol document provide by Eastron specify the commands and registers, but the timing (important) is specified in the SDM630 ModBus protocol document.
+
+Connections 3xSDM120 <-> board:
+
+- VCC; +5V
+- GND; ground
+- DI; PB10, USART3_TX
+- DE; PB1 (shared with RE since one is active low while the other is active high)
+- RE; PB1 (shared with DE since one is active low while the other is active high)
+- RO; PB11, USART3_RX
+
 ESP8266
 -------