EVLSERVO1 RS485 Termination: Can I keep board resistors with a terminated encoder?

[eric_m]

Hello all,

I am planning to purchase the EVLSERVO1 to control an industrial three-phase PMSM servo motor (rated 220V AC) for a hobby project. I will be running it at a lower voltage and accept the speed loss.

The motor has a 17-bit absolute encoder using RS485 physical layer (protocol is currently unknown, likely proprietary, which I plan to reverse engineer).

The EVLSERVO1 manual states: "The 120 Ohm termination resistors R84/R85 must be removed if termination is embedded at encoder side." My encoder has a built-in 100 Ohm termination resistor.

While I understand the manual's instruction to prevent double termination, I would prefer not to modify a new board if possible.

If I leave the resistors on the board, the parallel combination results in a ~54.5 Ohm load. Given that I plan to use a short cable (< 1 meter), is the EVLSERVO1's transceiver robust enough to drive this lower impedance without signal collapse? Or is the "must remove" warning critical because the board uses 3.3V logic and cannot source enough current for ~54 Ohms?

Any advice is appreciated!

[GMA]

Hello @eric_m,

Refer to "5.4.3 Absolute encoder" chapter of the EVLSERVO1 user manual.

[eric_m]

I noticed that for SPI and Full-Duplex, it specifically says to remove the termination resistors.

But for Half-Duplex (Z+/Z-), it doesn't say to remove them. Since the board acts as a Receiver in half-duplex, I'm assuming R85 should actually stay on the board to terminate my end of the bus?

If that's the case, and since my encoder is likely half-duplex anyway, it sounds like I can just leave the resistor alone and save myself the trouble of desoldering it. Does that logic hold up?

[GMA · Accepted solution]

Hello @eric_m,

About these constraints:

1. 54 Ohm is the lowest resistance value allowed by STR485E device
2. The current capability of LDO regulator inside STSPIN32G4 is 150mA.

Consequently, the parallel resistor of 54.5 Ohm is feasible for STR485E, although very close to the limit.

Assuming the encoder is supplied by VENC = 5V (default), a rough estimate of LDO consumption is STSPIN32G4 30mA – 60mA (depending on MCU core and peripheral usage) + STR485E U8 61mA + U9 28mA (without removing other termination resistors)

Therefore, the LDO seems very close to be overloaded.

However, the STSPIN32G4 is fully protected, and the case can be safely tested without hardware changes.

A thermal shutdown or device reset could occur during testing if exceeding LDO performance.

[eric_m]

Thank you very much for your detailed reply.

So if I understand correctly, powering the encoder from an external 5v supply would eliminate the overload risk, since the LDO would no longer be supplying power to the encoder.