Comtech EF Data CLC-10 - Manual (Page 37)

CLC-10 Comtech LPOD Controller Handheld Terminal M&C Accessory Revision 3
Serial-Based M&C using the CLC-10 MN-CLC10
3–5
3.4 Remote Control Protocol and Structure
The electrical interface is either an EIA-485 multi-drop bus (for the control of many devices) or an EIA-232 connection (for the control of
a single device), and data is transmitted in asynchronous serial form, using ASCII characters. Control and status information is
transmitted in packets, of variable length, in accordance with the structure and protocol defined in later sections.
3.4.1 EIA-485
For applications where multiple devices are to be monitored and controlled, full duplex (or 4-wire) EIA-485 is preferred. Half duplex (2-
wire) EIA-485 is possible, but is not preferred.
In full-duplex EIA-485 communication there are two separate, isolated, independent, differential-mode twisted pairs, each handling
serial data in different directions. It is assumed that there is a ‘Controller’ device (i.e., the CLC-10), which transmits data in a broadcast
mode, via one of the pairs. Many ‘Target’ devices (i.e., the SSPA) are connected to this pair, which all simultaneously receive data from
the Controller. The Controller is the only device with a line-driver connected to this pair – the Target devices only have line-receivers
connected.
In the other direction, on the other pair, each Target has a Tri-Stateable 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, in a framed ‘packet’ of data, the address of the intended
recipient Target is included. 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 Pair has one line driver (Controller), and all Targets have line-receivers.
Target-to-Controller pair Pair has one line receiver (Controller), and all Targets have Tri-State drivers.