Add function to read slave id

Author: Channel59

src/ModbusClient.cpp

@@ -163,6 +163,19 @@ long ModbusClient::inputRegisterRead(int id, int address)
   return value;
 }
 
+long ModbusClient::slaveIdRead(int id)
+{
+  uint16_t value;
+   
+  modbus_set_slave(_mb, id);
+
+  if (modbus_report_slave_id(_mb, 1, &value); < 0) {
+    return -1;
+  }
+
+  return value;
+}
+
 int ModbusClient::coilWrite(int address, uint8_t value)
 {
   return coilWrite(_defaultId, address, value);

src/ModbusClient.h

@@ -82,6 +82,15 @@ class ModbusClient {
   long inputRegisterRead(int address);
   long inputRegisterRead(int id, int address);
 
+    /**
+   * Perform a "Read Slave ID" operation for a single slave id.
+   *
+   * @param id (slave) id of target, defaults to 0x00 if not specified
+   *
+   * @return slave id on success, -1 on failure.
+   */
+  long slaveIdRead(int id);
+
   /**
    * Perform a "Write Single Coil" operation for the specified address and
    * value.