Modbus-RTU speed and delays

[Didro]

Hello, We use the FX3U PLC and a several Modbus devices: six TesysU Schneider-Electric Contactors with LULC033 communication modules, FR-E700 and FR-F700 Mitsubishi Inverters. For PLC communication with devices by Modbus-RTU procotol we use FX3U-485ADP-MB module and RS2 command. As written in the datasheet LULC033 have 19200 bps transmission speed. AFAIK inverters' maximum transmission speed is 19200 bps too. The problem is too big delay in communication. We have to wait for a modbus device answer for 300 milliseconds. If we try to read the Modbus receive register early (for example with period euals 200-250 ms) - we see that there is no answer. What the standart answer time for Modbus device and does we achieve speed limit ? Thank you

[kaare_t]

300ms is pretty much! Is this time the same in all the nodes, or just some of them? In any case, you have to wait for a reply before you send a new request. I've used FR-F700 and Modbus and they had about 40-80ms response time for each inverter, which is in my opinion pretty slow compared to other devices, but far from 300ms!!