Comtech EF Data PCB-4000 - Manual (Page 49)

5–1
Chapter 5. SERIAL REMOTE CONTROL
5.1 Overview
Remote monitor and control of the PCB‐4000 1+1 Phase Combiner is available either through 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.
5.1.1 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 communications, 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‐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, 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.