Black Box Black Box network services, PSD-4, PSD-8 2. Introduction

2. Introduction

The Programmable Sharing Device enables up to 4 (TL304A) or 8 (TL305A) modems or terminals to share

a master modem, multiplexor, or computer port in an asynchronous or synchronous multipoint environment.

(Asynchronous and synchronous equipment cannot be mixed on the same PSD.)

MAIN CHANNEL

The main channel transmits information to all sub-channels in parallel.

SUB-CHANNELS

Sub-channels contend to transmit to the main channel by activating RTS/DCD (Request to Send/Data Carrier

Detect) or by data transitions (strap-selectable). When the sub-channel is active, the sub-channel’s transmit data

and control signals are connected to the main channel. When RTS/DCD drops, data transitions stop, or 16 idle

bits are received, the PSD will disconnect and monitor other sub-channels.

Sub-channels can be disabled either manually by front-panel pushbuttons or automatically. If a sub-channel

is streaming and staying active for longer than a configured amount of time, the PSD will automatically disable

that sub-channel. A front-panel LED will illuminate to indicate that the sub-channel has been automatically

disabled. The sub-channel will be reset when RTS/DCD drops or 16 idle bits are transmitted to it from the

attached device.

CLOCK MODES

Three clock modes are supported: internal, external from the main channel, or external from sub-channel 1.

BUFFER

A built-in-buffer compensates for phase differences between the clocks of modems connected to the sub-

channels and the PSD main-channel transmit clock.

An extra-buffer option on the PSD-8 can be activated when the PSD is connected to equipment that cannot

accept an external clock. Examples include DDs (United States) or other digital lines and modems that cannot

be set to an external clock. This additional buffer compensates for phase differences between the PSD clock

and sub-channel clock(s).

Figure 1. The Programmable Sharing Device allows multiple devices to share a master port.

RS-232 Cable

Modem

CSU/DSU

PSD-4

AS/400®

Dialup Line

Leased Line

Modem

CSU/DSU

Controllers