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 ?
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.
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.
pauloynski said:
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).
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.
pauloynski said:
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 ?