UltraPort Serial Adaptors User Guide
The EIA-485 Standard
Page 135
Many Masters /
Many Slaves
The EIA-485 Full Duplex mode supports many Masters and many Slaves. This system can
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.
