Comtech EF Data CRS-300 - Manual (Page 218)

CRS-300 1:10 Redundancy Switch MN/CRS300.IOM
Serial-based Remote Product Management Revision 19
7–2
In the other direction, on the other pair each Target has a tri-state line driver connected, and the Controller has a line-receiver
connected. All the line drivers are held in high-impedance mode until one (and only one) Target transmits back to the Controller. Each
Target has a unique address, and each time the Controller transmits, the address of the intended recipient Target is included in a
framed 'packet' of data. All of the Targets receive the packet, but only one (the intended) will reply. The Target enables its output line
driver and transmits its return data packet back to the Controller in the other direction, on the physically separate pair.
EIA-485 (full duplex) summary:
• Two differential pairs – one pair for Controller-to-Target, one pair for Target-to-Controller.
• Controller-to-Target pair has one line driver (Controller), and all Targets have line-receivers.
• Target-to-Controller pair has one line receiver (Controller), and all Targets have Tri-State drivers.
7.3 EIA-232
This is a much simpler configuration in which the Controller device is connected directly to the Target via a two-wire-plus-ground
connection. Controller-to-Target data is carried, via EIA-232 electrical levels on one conductor, and Target-to-Controller data is carried
in the other direction on the other conductor.
7.4 Rules for Remote Serial Communications with the CRS-300
1) Always wait for a response (up to 15 seconds) from the CRS-300 before sending the next query or command.
2) If a "time-out" response ('~') is sent from the CRS-300, the user must resend the previous command. The ‘~’ response indicates
that a pass-through command to a modem/transceiver attached to the CRS-300 has "timed-out" and there was no response from
the other device. During this wait, do not communicate with the CRS-300. After the '~' response is sent by the CRS-300, it is now
ready to receive a message again. The CRS-300 knows to wait different times for the different messages it is sending to modems:
• Status queries (no commands) are fast, (typically less than 333 ms).
• Configuration changes (commands) take longer and vary by modem type.