document PZEM-004T connection

README.md

@@ -52,6 +52,7 @@ DDM100TC
 This 3-phase 4-wire electricity meter provides an S0 interface.
 
 On my model this would output 1600 impulse/kWh, thus each impulse corresponds to a power consumption of 0.625 Wh).
+This rate is hard coded in `main.c`.
 To record these impulses a timer input capture is used.
 The number of impulses determines the energy used.
 It is stored in a backup register, keeping the value in memory even when the board is powered down as long as a (coin cell) battery is connected to VBAT.
@@ -61,7 +62,27 @@ The time has a resolution of 0.91 ms (leading to a power calculation error of <1
 Connections DDM100TC <-> board:
 
 - S0+, pin 8; +3.3V or +5V
-- S0-, pin 7; TIM4_CH1, PB6 (use pull-down resistor)
+- S0-, pin 7; PB6, TIM4_CH1 (add pull-down resistor)
+
+PZEM-004T
+---------
+
+This 1-phase 2-wire electricity meter provides a UART interface.
+For the 3-phase 4-wire power distribution installation I used 3 meters, one per phase.
+
+Because each command includes a device address, they can be connected to the same UART port (one individual addresses have been set).
+All meters are periodically (see RTC) sequentially queried for their measurements (voltage, current, power, energy).
+The used addresses are hard coded in `main.c`
+A timer is used to guarantee a minimum (undocumented in the specification) time between requests in order to improve the response success.
+
+Connections 3xPZEM-004T <-> board:
+
+- 5V, 1; +5V (+3.3V is not sufficient)
+- RX, 2; PA2, USART2_TX
+- TX, 3, PA3, USART2_RX
+- GND, 4; ground
+
+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.
 
 ESP8266
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