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This section explains how the Model 378S handles flow control,
describes input/output cable connections, and tells how to operate the
Model 378S.
4.1 FLOW CONTROL CAPABILITIES
The Model 378S supports both software and hardware flow control
on all ports. There is no need to configure the switch for one method or
the other. However the input (PC) ports operate differently than the
output (printer) port with respect to flow control.
The selected PC and the printer never communicate directly.
Though it "appears" to the devices that they are communicating with
one another, each is actually communicating only with the Model 378S.
The Model 378S manages this communications traffic in the following
manner:
1) The Model 378S flow controls incoming data from the selected
PC by controlling CTS/DSR (hardware handshaking) or XON/XOFF
(software handshaking). PCs cannot flow control incoming data from
the printer or from the Model 378S.
2) The Model 378S retains unlimited use of the printer by keeping
RTS constantly high.
3) The printer flow controls data from the Model 378S using CTS
or XON/XOFF.
4.2 INPUT CABLE CONNECTIONS
All input ports are configured as DCE (Data Communication
Equipment): They are designed to connect directly to a PC or another
DTE (Data Termination Equipment) device using straight through
cables . If you are using XON/XOFF handshaking, then the only
required pins are pins 2, 3, and 7. If you use hardware handshaking,
then pin 5 or 6 must also be connected. The input port signals and
pins are shown below:
PIN
SIGNAL
DESCRIPTION
2
TD
Data INPUT to Model 378S from PC
3
RD
OUTPUT from 378S to PC (used
for XON/XOFF flow control only)
(continued)
5
5
CTS
OUTPUT from 378S to PC: Internally
connected to pin 6. Held low--toggles high
when port is chosen
6
DSR
OUTPUT from 378S to PC: Internally
connected to pin 5. Held low--toggles high
when port is chosen
7
SG
Signal ground
8
CD
OUTPUT from 378S at +V to PC
4.3 OUTPUT CABLE CONNECTIONS*
The output port is configured as a DTE: It is designed to connect
directly to a serial printer or plotter. The output port pin descriptions
are shown below:
PIN
SIGNAL
DESCRIPTION
2
TD
Data OUTPUT from Model 378S to printer
3
RD
INPUT to 378S from printer (used for
XON/XOFF flow control only)
4
RTS
OUTPUT from 378S at +V to printer
5
CTS
Flow control bus connection from printer
(used for CTS/DSR flow control only)
7
SG
Signal ground
8
CD
Not used
20
DTR
OUTPUT from 378S at +V to printer
*Note: The output (printer) port of the Model 378S, being configured
as a DTE, requires a female to male crossover cable pinned as fol-
lows:
Connection to 378S
DB-25 Female Pin No.
2 .....................................................3
3 .....................................................2
4 .....................................................5
5 .....................................................20
7 .....................................................7
20...................................................6 & 8
Connection to Printer
DB-25 Male Pin No.
6
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