Low latency serial communication on Linux

Question

I\'m implementing a protocol over serial ports on Linux. The protocol is based on a request answer scheme so the throughput is limited by the time it takes to send a packet

Answer 1

In short: Use a USB adapter and ASYNC_LOW_LATENCY.

I've used a FT232RL based USB adapter on Modbus at 115.2 kbs.

I get about 5 transactions (to 4 devices) in about 20 mS total with ASYNC_LOW_LATENCY. This includes two transactions to a slow-poke device (4 mS response time).

Without ASYNC_LOW_LATENCY the total time is about 60 mS.

With FTDI USB adapters ASYNC_LOW_LATENCY sets the inter-character timer on the chip itself to 1 mS (instead of the default 16 mS).

I'm currently using a home-brewed USB adapter and I can set the latency for the adapter itself to whatever value I want. Setting it at 200 µS shaves another mS off that 20 mS.