[15 pts for real help] Is the USB Power supply enough ?

[mouzid]

Hi edaboarders,
I have an USB to RS485 converter based on FT232 with max485 and I'm planning to use it in a RS485 network. The converter is self powered and dont need any external power supply. My question is:
Is the USB capable of powering the converter in a 1 Kilomerter network ?
Are there risks to be aware of ?

 

[pauloynski]

The MAX485 output short circuit current can go up to 250mA which is too much to be driven from the USB supply (250mA = 2.5 unit loads) supposing it is a FUNCTION- low bus-powered. No problem if it is a FUNCTION- high bus-powered (can sink up to 5 unit loads). If outputs are never shorted it is enough. The FT232R datasheet shows such an application. To reach 1km with the RS485 you may consider not using full speed.

 

[nandhu015]

Is the USB capable of powering the converter in a 1 Kilomerter network ?

I think it is not possible to power a device at 1km distance

Nandhu

 

[pauloynski]

Please let things to be clear: the RS485 network can reach 1km. To power the RS485 network at 1km is nuts.

 

[mouzid]

Thank you all for your replies.

Is the USB capable of powering the converter in a 1 Kilomerter network ?

I think it is not possible to power a device at 1km distance
Nandhu

Actually the converter is to be tied to the USB port. The outut of the converter is tied to a 1 Km twisted pair. At the other end of this wire I have a microcontroller based automata (Pic+max485) that uses an external power supply. The communication is unidirectional from the PC to the automata.

The MAX485 output short circuit current can go up to 250mA which is too much to be driven from the USB supply (250mA = 2.5 unit loads) supposing it is a FUNCTION- low bus-powered. No problem if it is a FUNCTION- high bus-powered (can sink up to 5 unit loads).

Could you please explain the concept of unit loads and how to choose between the low and high powered FUNCTIONs ?

If outputs are never shorted it is enough. The FT232R datasheet shows such an application.

Are you refering here for the FUNCTION- high bus-powered ?
My outputs are never shorted of course but you're right it's better to be aware of this risk and its impact on the HW. I decided to choose the function low bus powered function.

To reach 1km with the RS485 you may consider not using full speed.

How much the full speed and low speed datarate is ?
I was told that the converter is capable to transfer data at speeds up to 3M Baud in the RS485 network. Can the low speed support that ?

 

[pauloynski]

An unit load is defined as being 100 mA.
See http://www.connectworld.net/usb.html

RS485 reach: **broken link removed**