OC & OV bus protection

[mmx01]

Hi,

I am working on a gateway device that translates TTL to CAN & RS485/modbus frames. Software wise I have it all sorted but I also wanted to implement basic hardware protection.

Drivers:
CAN - SN65HVD230DR
RS485 - THVD1406DR
TTL - ESP32

Input voltage to the board is 48V with few step down DC/DC converters so I want to protect the circuit also against mistake in wiring on top of other failure related OV/OC conditions. Reverse polarity is addressed via Shottky diode but now I want also to protect bus drivers. I did not go for opto isolation and wanted to start with basic "minimum".

My plan is to use Resettable Fuses - PPTC on each line CL, CH, A+, B-, RX, TX for a total of 6 for OCP with trip current of 300mA/hold 100mA and 60VDC.

Then "crowbar" circuit for OVP:
>SZNUP2105LT1G one for CAN, CH, CL tied to GND
>SZNUP2105LT1G one for RS485, A+, B- tied to GND
>2x TISP4015L1AJR-S, one for TX and one for RX lines tied to GND.


Question 1: SZNUP2105LT1G is specificaly for CAN but looking at the datasheet no reason it wouldn't work for RS485. Brekadown voltage appears to conform with both. Do I miss something? Same part, simplier BOM.
Question 2: TTL operates at lower voltages, and breakdown voltage of SZNUP2105LT1G of 26-32V to my liking seems to be too far from the TTL levels. TISP4015L1AJR has breakover of 15V. Is that still too far off or acceptable? TVS diode vs. thyristor I don't consider a factor here, both parts are for digital communication so speed should not be a concern. CAN is 500k, RS485 is 9600 and TTL is 9600.

Thanks in advance.

Regards,
M

[Terry Bites]

RS485 can have legitimate CMVs ranging from  –7V to +12V. To be fully compliant you will need to accomodate this.
I've used 74S1053 bus terminators (with series resstors) to clamp over voltages. They can be used to set different clamping levels via Vcc and ground connections.

[mmx01]

So 2x TISP4015L1AJR-S on RS485 bus would offer better compatibility? This delivers -15V to +15V.

What puzzles me is all these parts state they are bidirectional, my assumption with SZNUP2105LT1G was that the breakdown voltage of 26V is truly from -26V to 26V so covering the spec. I am not sure of this assumption hence the question here.