2 and 4-wire resistance measurement

Whether a measurement is executed as a 2 or 4-wire measurement is determined by the connection points at which the measurement takes place. A comparison value is stored in the firmware for both measuring methods.

With the 2-wire measurement

a current is applied between the contact points RL+ und RL- and the voltage drop is measured in order to determine the resistance.
The parasitic line resistance cannot be determined by the box itself, but must be entered as a correction value in the CoE register 0x80n0:1B.

With the 4-wire measurement

the sensor current is applied between the contact points RL+ und RL- of the M12 socket and the voltage drop at the contact points R+ and R- is used to measure the resistance.
The conducting wire is thus not part of the measuring circuit and is not incorporated into the measurement as a source of error.

2 and 4-wire resistance measurement 1: — Resistance measurement with a 4-wire, 3-wire and 2-wire connection technique

The box uses the following calculation rule:

2 and 4-wire resistance measurement 2: — Data flow: resistance measurement with a 2 and 4-wire connection technique

With the values:

	Index in the CoE directory with n: channel number with 0 ≤ n ≤ 3 (channel 1 - 4)
X: Raw value	0x80nE:01
	Pt100		Pt1000
	2-wire	4-wire	2-wire	4-wire
Gv: Vendor Gain	0x80nF:04	0x80nF:06	0x80nF:0A	0x80nF:0C
Ov: Vendor Offset	0x80nF:03	0x80nF:05	0x80nF:09	0x80nF:0B
Y_R: Output value in 1/256 Ω	0x80nE:02

Overflow Y_R after 16 bits

This value is only for fault finding. The register overflows after 16 bits, i.e. at 65536.