CHAPTER 2: Introduction
9
2.1 Typical Synchronous Application
In this kind of application, the devices sharing the line (the “slave devices”)
are usually controllers at a remote site. The shared line runs from the RS-232
Data Sharer to a modem (the “master device”) that is ultimately linked to a
host computer. The controllers must be configured for multipoint modems,
with the Request to Send (RTS) signal switched. RTS has to be switched
because if a controller keeps RTS high, the Sharer will think the controller is
streaming and deny it access to the line (see
Section 4.5
).
Suggested jumper settings for a Sharer in this type of application:
√
Jumper W2 in A–B position (0 ms RTS delay)
√
Jumper W3 in A–B position (RXD Broadcast Mode)
√
Jumper W4 (4- and 8-Port Sharers only) in A–B position (sequential
polling)
√
Timeout-period jumpers in C position (timeout disabled); settings of
timeout-signal jumpers irrelevant.
The RS-232 Data Sharer is transparent to communications protocol (SDLC,
BSC, etc.), data-encoding technique (NRZ, NRZI, etc.), and addressing.
IBM
Mainframe
Modem
Modem
Leased
Line
IBM
Controllers
(5294, 3274, etc.)
Figure 2-1. Synchronous application—an RS-232 Data Sharer
in an IBM
®
multipoint environment.
RS-232 Data
Sharer-4