ITEM: D9358L
Is it possible to force encapsulation in transparent printing?
Question:
Model : 530 AIX : 3.2.4
The customer is trying to do terminal attached printing using a Texas
Insturments 924. This terminal sents output in 134 char. blocks. The
terminal is connected to a 128 port controller. The customer
doesn't want the printouts to come with the escape codes at the
beginning and the end of the printout. He does want the escape codes
to be put in if the buffer reaches it's limit. He has tried
changing the colon file and he has tried changes in smitty. These
changes do not seem to have much of an effect. Is this possible?
Response:
TI terminals have a hard coded limit of 134 consecutive characters being sent
to the aux port on the terminal. The RISC doesn't guarantee this. The
problem is, the terminals need a data stream like the following.
\-----max 134 char--\\---134 char---
Anything larger than 134 characters is too long.
Data sent to the transparent print device is broken into chunks
based on maxcps (Maximum characters per second), maxchar (Maximum
characters per block) and bufsize (estimated printer buffer size). This
happens whether the terminal is active or not. Notice I didn't say
encapsulated with onstr/offstr escape sequences.
Two things that affect encapsulation:
The terminal device is the master device, meaning when it has data
ready to send, the print device is stopped and terminal data is sent.
When terminal data has completed, transparent print data starts up again.
The transparent print onstr/offstr strings are only sent when making
the transition between terminal data and print data.
For example: The onstr is sent for the first block of print data, but
the offstr is not sent at the end of the data unless there is terminal
data to be sent. If the terminal is not active during the printing of
the transparent print job, the offstr will not get sent until the terminal
activity resumes (which could be after the print job has completed). This
is so we are not constantly switching the terminal in and out of print
mode if we don't have to. The terminal data does not go out the port in one
big burst, but in small packets defined by the maxcps, maxchar and bufsize
parameters even though onstr/offstr sequences are not being sent.
Hook it up and watch it work with a data scope sometime.
A note on the TI terminals:
Texas Instruments model 931, 924 and 928 async terminals support a "positive
acknowledgement" protocol to support VDT attached printers as true system
printers. This protocol requires terminal firmware support and tty device
driver support. The basic mechanism is described below.
The terminal firmware includes a data buffer dedicated for data
destined for the BDT attached printer. A start print data
"startp" escape sequence is sent to the BDT immediately preceding
printer data. So far no difference from IBM transparent print
mechanisms. However, a special escape sequence is sent to terminate
the printer data. Their terminating escape sequence is a command to the
terminal to send a "positive acknowledgement" escape sequence back
to the host (tty driver) when the terminal printer buffer has been
emptied (all the printer data has been sent to the printer). The tty
driver knows the size of the terminal print buffer (typically 132
bytes). The tty driver breaks print data into blocks of 132 or
less bytes. It sends one printer data block to terminal and requests
a positive acknowledgement when the terminal print buffer is empty.
The tty driver will never send the next print data block until a
positive acknowledgement sequence is received from the terminal.
The printer can be turned offline, run out of paper, jam, etc, without
affecting terminal operation and no print data will be lost.
This was a popular feature for Texas Instrument customers. Many
users connected all but their very high speed (>38400 bps), high
duty cycle printers to terminal auxiliary ports. This saved
system async ports and reduced cabling costs.
The above description was included to explain how the Texas Instruments
terminals work.
Now for the issue at hand. The Texas Instruments terminals support
multiple escape sequences to frame printer data which is sent to the
terminal. I suggest using the escape sequences that do not request a
positive acknowledgement sequence. You need to find the proper escape
sequence by reading the terminal manual. Using these escape sequences
will make the TI terminal work similar to other terminals.
Support Line: Is it possible to force encapsulation in transparent printing? ITEM: D9358L
Dated: October 1993 Category: N/A
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