Perle Systems 5500152-23 manual

Many Masters / The EIA-485 Full Duplex mode supports many Masters and many Slaves. This system can

Many Slaves be used when all EIA-485 devices have separate transmit and receive channels. There is no multiplexing of the TXD and RXD signals on the same device. This system is especially useful when there is no flow control available on the PC, usually due to the use of third party communications programs that prevents the use of the RTS signal as a “transmit enable” control. The advantages of this system are great, since no new communications software is needed, and the PC can talk and listen at the same time. In effect the handshaking is done by the intelligence of the UltraPort SI card.

As wired in the next diagram, the PC can transmit data at any time and the EIA-485 devices#1 to #n can simultaneously receive it. Only one of the EIA-485 devices may transmit data at any one time. Each EIA-485 device recognizes commands and data addressed to it and will only transmit when the Master PC commands it to do so. When the EIA-485 devices receives a command, the UltraPort SI card automatically turns its TXD drivers on and sends the data on the TXD channel, and then disables its TXD drivers afterwards. The other EIA- 485 devices remain in the tri-state mode when they are not being addressed and do not transmitany data.

UltraPort Serial Adaptors User Guide

The EIA-485 Standard

Page 135