Comtech EF Data CPA - Manual (Page 101)

7–1
Chapter 7. SERIAL-BASED REMOTE PRODUCT
MANAGEMENT
7.1 Overview
The serial-based remote product management interface for the CPA C-Band ISSPA consists of an EIA-485 multi-drop bus for the control
of many devices. 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.
The default port is EIA-485 (4-wire); selection is made via the front panel CONFIG: Remote  Serial  Interface menu. See Chapter
5. FRONT PANEL OPERATION for further information.
7.2 EIA-485
For applications where multiple devices are to be monitored and controlled, a full-duplex (or 4-wire plus ground) EIA-485 is preferred.
Half-duplex (2-wire plus ground) 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 a 'Controller' device (a PC or dumb terminal) transmits data in a broadcast mode via one of the pairs. Many 'Target'
devices are connected to this pair, and all simultaneously receive data from the Controller. The Controller is the only device with a line-
driver connected to this pair – the Target devices have only line-receivers connected.
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