Concept
General Legrand Power Meter Properties
An important difference between the Legrand meters is that models 04677, 04680, and 04684 use transceivers with integrated failsafe receivers, whereas the model 14669 uses a transceiver with no integrated failsafe function.
Review of the RS-485 interface provided by the Legrand meters yields the following interface attributes:
|
Legrand Model |
Transceiver |
Transceiver Unit Load |
Transceiver Failsafe |
Meter Circuit Bias |
Circuit Bias Load |
Total Unit Load |
Isolated RS-485 Bus |
|
04677 |
Exar SP4082 |
0.125 |
Yes |
10 kohm |
1.200 |
1.325 |
Yes |
|
04680 |
TI SN65HVD3082 |
0.125 |
Yes |
10 kohm |
1.200 |
1.325 |
Yes |
|
04684 |
TI SN65HVD3082 |
0.125 |
Yes |
10 kohm |
1.200 |
1.325 |
Yes |
|
14669 a |
Exar SP485E |
1.000 |
No |
16 kohm b |
0.750 b |
1.750 |
Yes |
- The RS-485 interface on the model 14669 is provided by the model 14673 plug-in option module.
- The 16 kohm network bias occurs with 14673 switches 1 and 2 in the OFF position, which is recommended. Placing these two switches in the ON position changes the bias to 8 kohm (1.5UL) and places a 120 ohm termination resistance across the network pair.
The provision of failsafe transceivers in the Legrand models 04677, 04680, and 04684 is a very beneficial feature. This type of transceiver with integrated failsafe receiver allows recommendation for a terminated bus without the negative limitation imposed with external bias resistors required to support termination. Configuration 1 supports these three meter models. For more information, see Legrand Configurations .
The model 14669 uses an older transceiver with no integrated failsafe feature and is rated for a full 1UL unit. This 1UL coupled with the internal bias resistance pushes the unit load of the model 14669 to a very high value of 1.75UL. This means each 14669 meter consumes almost two of the available 32 unit loads. Without the failsafe feature, the use of one of the alternate configuration options (2, 3, or 4) will be required. These configurations provide the required bias for the model 14669, but have the side effect of reduced node count and/or distance. For more information, see Legrand Configurations .
The models 04677, 04680, and 04684 can be used in any of the four configurations. If the network consists of only the models 04677, 04680, and/or 04684, Configuration 1 offers the best performance. If a single model 14669 is included on the network bus, the bus must use Configuration 2, 3, or 4 and follow the associated guidelines. For more information, see Legrand Configurations .
RS-485 Communications
Legrand Power Meters
Legrand Configurations