ISO1500DBQEVM: About Data rate vs Cable length

Hi Michael,

Thanks for reaching out. Please see below my responses to the questions you have listed out.

Michael Jeong1 said:

1. What's the max. cable length on my marking of below figure 33?

Please note that the plot shown in Figure 33 of ISO1500 datasheet is a representative typical cable length vs RS-485 datarate characteristic curve and not actual test data of ISO1500. This is included to show the typical inverse relationship between cable length and operating datarate. The actual maximum cable length achievable depends on the cable characteristics and overall system design.
The general acceptance of RS-485 standard is that it can supports cable lengths of upto 1200m (4000 ft) at low datarates. ISO1500 will also be able to support similar cable lengths.

Michael Jeong1 said:

2. Do you have a test result or simulation data about data rate vs cable length w/ ISO1500DBQEVM?

Sorry, we do not have test result or simulation data of datarate vs cable length for the EVM ISO1500DBQEVM. The results do not depend on EVM as the driving strength of device doesn't change with PCB. The achievable datarate for a given cable length depends largely on the cable used.

Michael Jeong1 said:

3. In condtion of max. 32node, cable length 1.2km and don't know cable type, what's the maximum supported data rate w/ ISO1500 in normally?

This is a question of the overall system design and not of ISO1500 alone. The whole system needs to be known for the achievable overall datarate to be estimated. For example, cable capacitance (affects rise/fall times) characteristics need to be know, cable length (which you have shared as 1.2km), wire gauge, acceptable rise/fall times as a percentage of bit width (customer needs to define this) and characteristics of other RS-485 nodes.

Assuming a cable capacitance of 100pF/m (120nF for 1.2km), cable length of 1.2km, suitable wire gauge for RS-485, 60Ω termination, rise/fall times of 30% bit width and assuming only ISO1500 devices on the bus, we can estimate the maximum datarate achievable to be as approximately 16kbps. Please note that this is only an approximation and the actual test results can be worse than these.

I hope these points answer your questions, thanks.

Regards,
Koteshwar Rao

Hi Michael,

Thanks for sharing customer requirements.

Like I mentioned earlier, the overall datarate that can be achieved is system dependent and is not a limitation of ISO1500. Most isolated RS-485 transceivers that comply to the RS-485 standard have similar drive strength and hence, they generally perform similar. The above calculation that I have provided is generic and is applicable to most RS-485 transceivers.

If customer wants to achieve higher datarate for given cable length, then they should choose a better cable with lower capacitance and they should be able to achieve higher datarate. This is the only feasible option I can think of.

Let me know if you have any further questions thanks.

Regards,
Koteshwar Rao