ECM-1240 -> GEM upgrade: keep existing Com-Dual Split-200 wiring?

[jnh]

I'm preparing to replace my trusty ECM-1240 with a GEM, and would prefer to make as few changes as possible to the existing CT wiring.

My question involves the relationship between a GEM's 4-terminal input blocks, and the ECM-1240's 3-terminal COM-Single-Dual Ch1/Ch2 (non-AUX) inputs. I have a pair of Split-200 CT's across the mains, wired directly to each other in series, with the remaining "outer" conductors (black of one CT, white of the other) connected to the Com and Dual terminals of my ECM-1240's Ch1, with its Single terminal vacant.

Can I retain this existing 2-wire connection, using pins 1 and 4 of a GEM input? (Or 1 & 2, or some other pair of terminals?) The manual says to wire one CT to 1 & 2, and the other to 3 & 4, but are those middle connections truly necessary, or just a convenience? All CT leads are extended out of this panel using a set of Cat5 cables, and using only 2 conductors for this channel rather than 4 would save me an extra Cat5 run.

How do the GEM's analog inputs actually connect to its 4-terminal blocks? I'm guessing it's a single 2-wire differential input per channel, but with different resistors in circuit depending on the pins used? Measuring input resistance, I get

1-2 or 3-4: 254 ohms
2 - 3 : 7 ohms
1 - 4 : 515 ohms

But I didn't think to check this on the ECM-1240 before it was installed ~7 years ago, and am not yet ready to take it out of service.

EDIT: I just noticed Figure 7 on page 6 of the "GEM Quick Guide", showing a series connection of Type-A (internal burden resistor) CT's to pins 1 & 4, so it looks like this will work as I'd hoped. Still, if anyone can confirm this with the Split-200's specifically, that would be appreciated. And do I need to make any configuration change, as compared to having each Split-200 on 1&2 or 3&4?

Thanks!

[ben]

I'm preparing to replace my trusty ECM-1240 with a GEM, and would prefer to make as few changes as possible to the existing CT wiring.

My question involves the relationship between a GEM's 4-terminal input blocks, and the ECM-1240's 3-terminal COM-Single-Dual Ch1/Ch2 (non-AUX) inputs. I have a pair of Split-200 CT's across the mains, wired directly to each other in series, with the remaining "outer" conductors (black of one CT, white of the other) connected to the Com and Dual terminals of my ECM-1240's Ch1, with its Single terminal vacant.

Can I retain this existing 2-wire connection, using pins 1 and 4 of a GEM input? (Or 1 & 2, or some other pair of terminals?) The manual says to wire one CT to 1 & 2, and the other to 3 & 4, but are those middle connections truly necessary, or just a convenience? All CT leads are extended out of this panel using a set of Cat5 cables, and using only 2 conductors for this channel rather than 4 would save me an extra Cat5 run.

It's a convenience factor. You can run them to 1 and 4 for the Splits.

How do the GEM's analog inputs actually connect to its 4-terminal blocks? I'm guessing it's a single 2-wire differential input per channel, but with different resistors in circuit depending on the pins used? Measuring input resistance, I get

1-2 or 3-4: 254 ohms
2 - 3 : 7 ohms
1 - 4 : 515 ohms

But I didn't think to check this on the ECM-1240 before it was installed ~7 years ago, and am not yet ready to take it out of service.

EDIT: I just noticed Figure 7 on page 6 of the "GEM Quick Guide", showing a series connection of Type-A (internal burden resistor) CT's to pins 1 & 4, so it looks like this will work as I'd hoped. Still, if anyone can confirm this with the Split-200's specifically, that would be appreciated. And do I need to make any configuration change, as compared to having each Split-200 on 1&2 or 3&4?

Thanks!

Ports 2 and 3 have the AUX resistor across for Micro-CTs.