DATAPROCESS ST 230 Manual do Utilizador descarregar pdf (Página 6)

ST 230ST 230

ST 230

PROGRAMMING GUIDE PROGRAMMING GUIDE

PROGRAMMING GUIDE

Page 6 Page 6

Page 6

The transmission medium for the ST230 network is a coaxial cable (typically the

“thin” Ethernet 50 ohm type, RG58 or equivalent) with BNC connectors and “T”

junctions linking together the computer, equipped with the appropriate interface on

the serial port, and up to 15 printers, each galvanically insulated from the others,

within maximum distances of several hundred meters.

Due to the half-duplex nature of the network, a master-slave communication

protocol has been implemented, where the computer (master) is always the

originator of any activity on the line and the printers (slaves) send data only when

requested.

The RTS and CTS lines are no more used for data-flow control, which now relies

only on the XON/XOFF mechanism: RTS activates the 5 MHz modem carrier

(carrier ON when RTS is at positive level), while CTS detects the presence or

absence of the carrier (carrier ON when CTS is at negative level). It’s responsibility

of the application program running on the external computer to check the presence

of the CTS (indicating that the line is free), then set the RTS before starting

transmission and reset it after the last character has been sent. Please note that the

network interface adapter automatically echoes back any character transmitted:

this must be kept in mind when designing the software driver.

Each printer connected on the network must have a unique network address

(indicated by the “NA=xx” field printed on the status label), ranging from 01 to 15,

that currently represent the maximum number of printers that can be connected to

the same network. Set-up of the network address is possible only through the front

panel of the printer, in a way similar to selection of baud rate and character map (see

the section describing SET-UP mode).

When operating in network mode (00<NA<16), the printer is by default inactive after

switch-on (it does not send any character and ignores received data) until it

recognizes the appropriate activation sequence composed of an ASCII control

character whose code is equal to its address plus 16 (codes 11 to 1F hex, 17 to 31

dec), followed by US-US-NAK (codes 1F-1F-15 hex, 31-31-21 dec).

When the printer recognizes its own activation sequence, it first sends all the

characters currently contained in its transmit buffer (transmission is triggered by the

low-to-high transition of the CTS line, indicating that the master has switched off its

carrier), terminating them with an EOT (ASCII code 4), then it enables the receive

buffer and begins behaving like a normal, non-networked printer, with the only

difference that any status information produced during printing (or when a status

request is processed) is buffered and it is not automatically transmitted until another

activation sequence is received. If the transmit buffer is empty when the activation

sequence is received, only the EOT is transmitted.

When the printer recognizes the activation sequence addressed to another printer,

it inhibits the receive buffer and ignores all subsequent incoming characters until

the next activation sequence addressed to itself.

This master-slave architecture requires a periodic polling of all the printers on the

network, particularly of those that are printing, to avoid saturation of their transmit

buffers, whose size (4 Kbytes) allows an autonomy of about 500 labels, after which

the machine suspends operation. Furthermore, when sending a lot of data to the

same machine and/or working in spooling mode, it is necessary to periodically

interrupt transmission and resend an activation sequence to the printer, allowing it

to notify possible overflows of the receive buffer by sending back XOFF characters.

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