How can I use SPEBlox Long with GigaBlox nano for a tethered robot

BotBlox Hardware
1

– Copied from customer email –

I am looking into using Bot blox switches for a tethered robot application.

Please see image below, The tether is up to 1km long and we require low speed communication for sensor and compressed camera data.

I think the 1km SPE switch paired with the Gigablox Nano 1gb switch would be suitable for this application.

I have the following questions:

  • In regards to the image below do you see any potential issues with this application?

  • The high density connector on the 1gb switch makes integration with your products very simple, do you have a similar product available for the SPE switch that includes a high density connector?

  • Ideally in this application we would only have 1 switch that converts SPE to 10 Base T and allows the 2 processors to communicate on the same network over 100 Base T. Do you have a product that combines these 2 together? If not, would you be able to put a solution together that combines the 2 boards onto 1 pcb with a high density connector?

  • For below application, do we require any specific connectors for the SPE? E.g. MagJack connectors or something similar.

  • What cable specifications do you recommend for SPE over 1km range?

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Thanks for sharing the diagram. Your setup looks good, and using 10BASE-T1L for the 1 km tether is a good approach for low-speed data such as sensor and compressed video. I’ve added some notes below on the main points and a few potential issues to be aware of.

  1. Application overview

The topology you’ve proposed (SPEBlox 10M → GigaBlox Nano → processors) is workable, but there is an interface mismatch to be aware of.

SPEBlox 10M converts between 10BASE-T1L and 10BASE-T. 10BASE-T is carried on four Ethernet pins, while the GigaBlox Nano’s ports are 10/100/1000BASE-T which uses eight pins.

Normally you can connect four pins of an eight pin ethernet interface, and the speed will auto-negotiate down to the next fastest speed available which is 100BASE-T.

However in this case you actually want it to go down to 10BASE-T so it is compatible with SPEBlox Long. This means fixing the port speed on GigaBlox Nano manually, something that isn’t possible on that board.

To make this work, you’d need to use a switch that allows you to manually set the port speed. SwitchBlox Industrial can do this. SwitchBlox Industrial version allows you to force the port mode to 10BASE-T, ensuring compatibility with the SPEBlox Long.

  1. High-density connector for SPE

At present, SPEBlox 10M doesn’t use the same high-density board-to-board connector found on GigaBlox Nano. In other words, it’s not mountable. We could release one but we normally like to look at potential order volumes before we consider that. Can you share that information?

  1. Combined SPE + switch option

We don’t currently offer a single board that combines the 10BASE-T1L interface and a multi-port Ethernet switch, but this is something we can support as a custom PCB. Again, if you could share a little more about your potential order volumes that would be helpful.

  1. Connectors and cabling

SPE side: Use a Single-Pair Ethernet (IEC 63171-6) connector or a shielded two-pin terminal if you’re terminating inside an enclosure.

100BASE-TX side: SPEBlox Long contains transformers onboard so you don’t need to add more magjacks. Same is true of SwitchBlox Industrial. With Gigablox Nano, there are no transformers, but since there are transformers on SPEBlox Long, and both GigaBlox Nano and SPEBlox Long are voltage mode, you could get away with not adding them to the GigaBlox Nano side. Though this is a moot point because GigaBlox Nano can’t be used with SPEBlox Long for the aforementioned reason.

Cable: For the 1 km run, use a shielded single-pair cable rated for 10BASE-T1L, typically 18–22 AWG, 100 Ω impedance, and low capacitance. Choose a jacket suitable for your environment (PUR/TPU for robotics tethers).

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I didn’t make the original enquiry, but I have a very similar application where I want to connect multiple devices at the end of a 400m twisted pair - so add +4 to the market demand and it seems like a common need to me. Josh mentions the Gigablox nano wont work as the switch because it can’t be forced to 10base-T. Is there the same incompatibility with the Standard SwitchBlox (same question as above but SwitchBlox instead of Gigablox)? I was assuming the SPEBlox-Long would set the rate.

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To be fair you can already achieve this using SPEBlox Long + SwitchBlox Industrial, but I know that isn’t the most elegant solution. What you’re asking for is an integrated SPE switch; that’s not something we’ve built yet.

SwitchBlox won’t work for the same reason as GigaBlox Nano; there is no way to set the port speed manually. This is only possible on our boards that run management software; currently this is only UbiSwitch, SwitchBlox Industrial and SwitchBlox Rugged.

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5

Thanks for confirming switch requirements for use with the SPEBlox Long. Does the connection between SPEBlox Long and SwitchBlox Industrial require a crossover cable, or can I use a 1 to 1 picoblade to picoblade cable?
To date I’ve only configured web-managed switches, but looks like I can fumble through the UART control with BloxOsLite. Do those settings persist through power cycles, or do I need to set the PHY Speed at every startup?

How sensitive is the achievable transmission length to twisted pair impedance? The 400m shielded twisted pair I will use between the SPEBlox Long’s has 80 Ohm impedance. Is that likely to work? I cannot change the twisted pair as it is part of a standard ROV tether design (that is more geared towards 75 Ohm composite video transmission).

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Thanks for confirming switch requirements for use with the SPEBlox Long. Does the connection between SPEBlox Long and SwitchBlox Industrial require a crossover cable, or can I use a 1 to 1 picoblade to picoblade cable?

With autonegotiation disabled, the pins will still do auto-mdi(x), meaning you can use a straight or croossover cable.

To date I’ve only configured web-managed switches, but looks like I can fumble through the UART control with BloxOsLite. Do those settings persist through power cycles, or do I need to set the PHY Speed at every startup

Configurations can be stored throughout power cycles using the port save command

How sensitive is the achievable transmission length to twisted pair impedance? The 400m shielded twisted pair I will use between the SPEBlox Long’s has 80 Ohm impedance. Is that likely to work? I cannot change the twisted pair as it is part of a standard ROV tether design (that is more geared towards 75 Ohm composite video transmission).

It’s difficult to give a definitive answer on that. 10BASE-T1L achieves 1000 meters on 28AWG, 100Ohm differential twisted pair so my general sense is that 400 meters will probably work just fine on 80 Ohm differential impedance. Certainly I’ve seen other customers achieve something similar. However the only way is to try it and see. It should work in my experience. Be sure to share your results here!

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