Emerson M21, M22 RS-485 Modbus RTU, Reg. Value Current Output Status LED Meaning Red Green

Table 6: Current output and meaning (cont’d)

Reg. Value		Current Output	Status LED		Meaning
		(mA)	Red	Green
	2.5		Fast Flash		Sensor Element Failure
15
					The concentration value is greater than the
16	20.0		Solid		allowed range; the sensor is displaying its
					maximum concentration value.
					FAULT DETECTED: Sensor baseline has
17	2.5		Fast Flash		drifted into a “Negative Gas Concentration”
					region (Zero Drift) and requires re-calibration.
18	4-20		Fast Flash		Sensor is nearing end of life. Replace at next
					calibration.
20	2.5		Fast Flash		FAULT DETECTED: A critical memory fault
					has occurred.
21	2.5		Fast Flash		FAULT DETECTED: A fault has occurred in
					the sensor’s onboard power supply.

Note: A slow flash is defined as the Status LED being ‘ON’ for 50 milli-seconds and ‘OFF’ for 1 second, while a fast flash is the LED being ‘ON’ for 250 milli-seconds and ‘OFF’ for 250 milli-seconds and a very fast flash is the LED ‘ON’ for 50 milli-seconds and ‘OFF’ for 50 milli-seconds.

5.4 RS-485 Modbus RTU

RS-485 Modbus RTU protocol is used. Tables 7 and 8 on the following pages give the detailed MODBUS registers and the bit values for the transmitter register.

The Millennium II Transmitter utilizes 2- wire Modbus RS-485 multi serial mode. This Modbus solution implements a 2-wire electrical interface in accordance with the EIA/TIA-485 standards. For this MODBUS configuration, it is important that a third wire be used for connecting all the ‘Common’ (COM) in the chain. Also a 120 Ohm line termination is required for the last device in the line. See Figure 19. The Instrument Engineer is responsible for calculating line length and adhering to MODBUS protocols.

Figure 19: Two (2)-Wire Modbus configuration

37

MAN-0076 Rev 05 Millennium II December 07, 2012

Net Safety Monitoring Inc