Programming
56
025-9368H
XON / XOFF PROTOCOL
The Model 748 follows “XOFF/XON” protocol. This sequence prevents data from flowing too
rapidly for the display device (printer/terminal) or host to assimilate. If the Model 748 receives an
“XOFF” code (
Control - S
), the data output will pause until an “XON” code (
Control - Q
) is received
or 20 seconds expire.
SERIAL INTERFACE CONNECTIONS
The local serial I/O port on the Model 748 is compatible with RS-232C signals and uses an
asynchronous ASCII serial communications protocol. Only three wires need to be connected from
your terminal to the serial I/O connector on the Model 748: Tx data, Rx data and Ground. Set the
terminal for 8 bits per character, no parity bit, and 1 stop bit (see the reference manual for the
terminal you are using). Set the terminal type or mode to ANSI or VT102. The Model 748 defaults
to 4800 baud. If nothing happens, try reversing pins 2 and 3 at the connector on your
terminal/computer.
CAUTION:
The 9 pin RS-232 connector on the Model 748 has two different pin out configurations. The jumpers
JP1 and JP2 must BOTH be in position
A
for Zetron, or BOTH in position
B
for AT. Default
configuration =
A
(Zetron).
Table 3. Model 748 DB-9 Pin-out
JP1, JP2 = ZETRON
INTERNAL CIRCUITRY
PIN
JP1, JP2 = AT
INTERNAL CIRCUITRY
NC
1
NC
NC
2
Tx data (transmit data)
Tx data (transmit data)
3
Rx data (receive data)
Rx data (receive data)
4
jumper to pin 6
Ground
5
Ground
jumper to pin 4
6
jumper to pin 4
jumper to pin 8
7
jumper to pin 8
jumper to pin 7
8
jumper to pin 7
NC
9
NC
The 9 pin RS-232 connector on the Model 748 can be configured two ways. In the default
configuration (JP1 and JP2 =
A
) provides compatibility with previous Zetron products (M38, M39,
M45, M48).
The alternate configuration (JP1 and JP2 =
B
) provides compatibility with standard “AT” DB9
connection by providing the null modem termination. This works with a standard DB9 extension
cable.
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