User's Manual

Models CH45
EIA 422 or 485
2-8
The Master must have its Mode Jumpers in the "OFF" position. When it transmits a request out its T
optical port, it will receive the echo of its request at its R optical port. This request has gone around the
loop, and has been repeated by each Slave in the loop. However, the Master does not repeat (re-
transmit) any of these received signals optically back around the loop, because its Mode Jumpers are in
the "OFF" position.
NOTE
The communication logic and control software of the Master IED must be able to manage the receipt
of its echoed request. In the FD Mode, the receipt of the echo can be used in conjunction with a
watchdog timer to continuously verify loop integrity.
When addressed, the Slave will transmit the appropriate response. Each Link/Repeater connected to a
Slave IED must have its mode Jumpers set in the "ON" position. In this mode, all signals received on a
Slave's R optical port are delivered to the IED’s communication port and at the same time repeated out
the T optical port to the next device in the loop. If an IED determines that this request requires a
response, then the Link/Repeater transmits the IED’s response out the T optical port. The response is
repeated at each Slave device, until it arrives at the Master.
When an IED is a Slave, it should not attempt to initiate a transmission while it is receiving a signal.
Since signals being received are also being repeated at the same time, any attempts to transmit its
response while still receiving will corrupt both transmissions due to a data collision.
CAUTION
If a Slave IED attempts to transmit while receiving a message, a
data collision will occur and both data packets will be corrupted.
In Master/Slave Loop Operation, half duplex communication (sequential transmit and receive functions) is
available. Only the Master must communicate full duplex (simultaneous transmit and receive) in a
Master/Slave loop, provided its IED has independent Transmit and Receive channels, and the
Link/Repeater HD/FD Jumpers is placed in the FD position.
CAUTION
If the Master IED operates in half duplex mode, special steps
must be taken to control the echo. (Refer to Section 3.6.)
APPLICATION NOTE
In a Master/Slave Loop Operation, the communication logic (control software) and the Master
IED must manage:
1) the transmission to Slaves (including addressing).
2) the receipt of the echo of its transmissions.
3) the receipt of the Slave's response to its transmission.
4) the control of the Slaves to prevent the initiation of a transmission while
receiving a signal.