Because formatted transfers require a formal protocol, a host computer used with the 8540 must be capable of
Fig. 4-6. Protocol for formatted downloads.
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satisfying that protocol. In particular, the following programs are required on the host computer:
1. A transmit program that will transfer Tekhex blocks to the 8540.
2. A receive program that will accept Tekhex blocks from the 8540.
3. If you have object files in non-Tekhex format that you would like to transfer and use on the 8540, you must have a conversion program that will translate the object files into Tekhex format.
Notice that without handshaking, the protocol for a formatted transfer is the same as for an unformatted ASCII transfer, as described eariier in this subsection. Therefore, a simple copy command could serve as a transmit or receive program. However, the 8540 generally takes longer to process a message block than the host requires to send the next one, so the transmit program must pause enough between lines that no data is lost.
One way for the host to achieve the necessary delay is to transmit "fill" characters at the beginning of each message block. COM finishes processing one message block while it receives (and ignores) the fill characters at the beginning of the next block. You may use any characters you want for fill, so long as they differ from the host's end-of-line characters(s) and from the leading character of the message block. For transfers at 2400 baud, four to six fill characters should be sufficient; the actual number required depends on your host and baud rate.
Transmit Program. This program downloads Tekhex-formatted files to the 8540, using the download protocol described earlier in this section. The general algorithm is as follows:
1. Read a Tekhex block from a file (error checking for valid format is optional).
2. Transmit the block followed by the predefined prompt sequence, and wait for an acknowledgement. The prompt sequence is defined in the COM command line, and the 8540 will not respond until it receives the prompt.
3. If the reply is a negative acknowledgement (NAK):
a. Increment the negative acknowledgement counter (this counter keeps track of how many successive transmission failures have occurred).
b. If the negative acknowledgement counter exceeds a maximum value that you have defined, terminate the transfer by sending an abort block.
c. Prepare to re-transmit the block that was incorrectly received and go to step 2.
4. If the reply is a positive acknowledgement (ACK):
a. If the block sent was a termination block, terminate the transfer.
b. Otherwise, go to step 1.
REV JAN 1983
Intersystem Communication-8540 System Users
This algorithm is presented in pseudo-code in Fig. 4-7.
Receive Program. This program receives Tekhex files from the 8540, using the upload protocol described earlier in this section. The general algorithm is as follows:
1. Read a Tekhex block from the 8540.
2. Verify proper format. A Standard Tekhex block must begin with a slash and must not exceed 71 characters in length (not counting the end-of-line). An Extended Tekhex block must begin with a percent sign and must not exceed 255 characters in length (not counting the end-of-line).
3. If the block is a termination block, a. Verify the checksum.
b. If an error is encountered, send a NAK followed by the pre-defined prompt sequence, and increment the NAK counter.
END-OF-FILE
=
FALSEWHILE NOT END-OF-FILE
READ TEKHEX BLOCK FROM FILE
Formatted Transfers
c. If no error is encountered, write the block to a file, transmit an ACK followed by the predefined prompt sequence, and exit the receive program.
4. If the block is not a termination block, a. Verify the byte count.
b. Verify the checksum(s).
c. If an error is encountered, increment the NAK counter and test to see if it exceeds a maximum value that you've selected. If it exceeds that value, terminate the transmission with an abort block;
otherwise, send a NAK followed by the predefined prompt sequence to the 8540. The prompt sequence is defined in the COM command line, and is used by the 8540 to determine when the host computer is ready for input.
d. If no error is encountered, write the block to a file and send an ACK followed by the predefined prompt-sequence to the 8540.
IF TEKHEX BLOCK IS A TERMINATION BLOCK THEN END-OF-FILE
=
TRUE NUMBER-OF-NAKS=
0GOODTX
=
FALSE REPEATTRANSMIT BLOCK
TRANSMIT PROMPT SEQUENCE GET REPLY
IF REPLY IS AN ACK THEN GOODTX
=
TRUE ELSENUMBER-OF-NAKS
=
NUMBER-OF-NAKS + 1 IF NUMBER-OF-NAKS >= 5END THEN
END
TRANSMIT ABORT BLOCK GOODTX
=
TRUE END-OF-FILE=
TRUEUNTIL GOODTX
=
TRUEEND EXIT
Fig. 4-7. Algorithm for Tekhex transmit program.
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FOimattad Transfers
This general algorithm is presented in pseudo-code in Fig.
4-8.
Conversion Programs. This discussion gives two algorith-ms for converting a file of object code into Tekhex format.
The first algorithm is for conversion to Standard Tekhex,
NAK-COUNTER
=
0END-OF-TRANSMISSION
=
FALSE WHILE NOT END-OF-TRANSMISSIONCLEAR ERROR FLAG
READ A TEKHEX BLOCK FROM THE RECEIVE LINE
IF TriE BLOCK STARTS WITH A n / n AND BLOCK-LENGTH <= (.1
OR THE BLOCK STARTS WITH A "%" AND BLOCK-LENGTH <= 255 THEN
IF BLOCK IS A TERMINATION BLOCK THEN
VERIFY CHECKSUM IF ERROR
THEN SET ERROR FLAG
intersystem Communication-8540 System Users
and the second is for conversion to Extended Tekhex.
Because of the wide variation in object file formats from assembler to assembler, it is possible to give only very general algorithms.
ELSE **NO ERROR IN TERMINATION BLOCK**
END
END
WRITE BLOCK TO FILE END-OF-TRANSMISSION
=
TRUE TRANSMIT ACK, END-OF-LINE TRANSMIT PROMPT SEQUENCEIF BLOCK IS A NOT A TERMINATION BLOCK THEN
VERIFY BYTE COUNT VERIFY CHECKSUM(S)
IF ERROR
THEN SET ERROR FLAG
ELSE **NO ERROR IN DATA OR SYMBOL BLOCK**
WRITE BLOCK TO FILE TRANSMIT ACK, END-OF-LINE TRANSMIT PROMPT SEQUENCE NAK-COUNTER
=
0END END END
ELSE **BLOCK FORMAT IS BAD**
SET ERROR FLAG END
IF ERROR FLAG SET THEN
NAK-COUNTER
=
NAK-COUNTER + 1END END EXIT
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IF NAK-COUNTER >= NAK-LIMIT
THEN **IF NUMBER OF NAKS EXCEEDS NAK-LIMIT,
TRANSMIT ABORT BLOCK ABORT THE TRANSMISSION**
END-OF-TF_~NSMISSION TRl~
END ELSE
END
TRANSMIT NAK, END-OF-LINE TRANSMIT PROMPT SEQUENCE
Fig. 4-8. Algorithm for Tekhex receive program.
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Standard Tekhex Conversion Program.
1. Discard any header information and scan for the first block of code.
2. Read the block of code.
3. If there are no more blocks:
a. Generate a termination block.
b. Stop execution.
4. Strip off any non-object code information from the block.
5. Check the block for valid input format.
6. Start the output block with a slash.
7. Immediately after the slash, append the 4-digit load address for the block.
8. Determine the byte count n, where n is the number of bytes of object code in the block. Place the byte count in the output block immediately following the load address.
DISCARD HEADER INFORMATION END-OF-FILE
=
FALSE WHILE NOT END-OF-FILEREAD BLOCK FROM INPUT FILE IF NO MORE BLOCKS
THEN END-OF-FILE
=
TRUE ELSEIF BLOCK IS A TERMINATION BLOCK THEN
Formatted Transfers
9. Calculate the first checksum as the sum of the individual hexadecimal digits of the load address and the byte count. Append this value immediately following the byte count.
10. Put the n bytes of object code into the block, immediately following the first checksum. While performing this step, calculate the second checksum as the sum, mod 256, of the 2n hexadecimal digits in the object code.
11. Write the block to the output file.
12. Go to step 2.
This general algorithm is presented in pseudo-code in Fig.
4-9.
SAVE TRANSFER ADDRESS FOR TEKHEX TERMINATION BLOCK END-OF-FILE
=
TRUEEND END
END ELSE
END
SAVE LOAD ADDRESS
STRIP NON OBJECT CODE INFORMATION FROM BLOCK CHECK FOR VALID FORMAT
IF ERROR THEN
OUTPUT ERROR MESSAGE END-OF-FILE
=
TRUE ENDELSE
END
OUTPUT-BLOCK [01
=
"1"DETERMINE LOAD ADDRESS
OUTPUT-BLOCK [1 .. 4]
=
4-DIGIT LOAD ADDRESS DETERMINE BYTE COUNTOUTPUT-BLOCK [5 .. 6]
=
2-DIGIT BYTE COUNT CALCULATE FIRST CHECKSUMOUTPUT-BLOCK [7 .. 8]
=
2-DIGIT CHECKSUM OUTPUT-BLOCK [9 .. 2n+8]=
OBJECT CODE CALCULATE SECOND CHECKSUMOUTPUT-BLOCK [2n+9 .. 2n+10]
=
2-DIGIT CHECKSUM WRITE OUTPUT-BLOCK TO FILEGENERATE TEKHEX TERMINATION BLOCK EXIT
Fig. 4-9. Algorithm for Standard Tekhex conversion program.
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Formatted Transfers
Extended Tekhex Conversion Program. This algorithm makes the following assumptions:
• The object file may contain program symbols and section information as well as executable object code.
• All the usable information in each block is output to the Tekhex file before the next block is read from the object file. (You may find it more useful to save the program symbols in a table as you proceed through the object file, and generate your symbol blocks from the symbol table at the end of the program.)
• At most one data block and/or one symbol block is generated for each block read from the object file: there are no checks to verify that the maximum block length (255) has not been exceeded.
• The variable n is used to represent the number of digits in a variable-length number. The value of n may be constant throughout the program, or it may be different for each number you encode (for example, if you choose to eliminate leading zeros). Similarly, the variable 5 represents the number of characters in a variable-length symbol.
Here is the algorithm:
1. Discard any header information and scan for the first block of code or symbols.
2. Read the block.
3. If there are no more blocks:
a. For every undefined section, generate a symbol block with a section definition field.
b. Generate a termination block.
c. Stop execution.
DISCARD HEADER INFORMATION END-OF-FILE
=
FALSEIntersystem Communication-8540 System Users
4. Strip off any extraneous information.
5. Check the block for valid input format.
e. Determine the block length and checksum and put them in the block header.
c. Place the name of the section that contains these symbols in the block. definition field containing the symbol, its type, and its value.
SAVE TRANSFER ADDRESS FOR TEKHEX TERMINATION BLOCK END-OF-FILE
=
TRUEOUTPUT ERROR MESSAGE END-OF-FILE
=
TRUEFig. 4-10. Algorithm for Extended Tekhex conversion program. (part 1 of 2)
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END END
ELSE
END END
IF BLOCK CONTAINS EXECUTABLE CODE THEN **GENERATE DATA BLOCK**
OUTPUT-BLOCK [oJ
=
"%"OUTPUT-BLOCK [3J
=
"6"OUTPUT-BLOCK [6J
=
nOUTPUT-BLOCK [7 .. 6+nJ = n-DIGIT LOAD ADDRESS OUTPUT-BLOCK [7+n .. 6+n+2mJ = m BYTES OF OBJECT CODE OUTPUT-BLOCK [1 .. 2J = 6+n+2m **2-DIGIT BLOCK LENGTH**
OUTPUT-BLOCK [4 .. 5J = 2-DIGIT CHECKSUM WRITE OUTPUT-BLOCK TO FILE
END
IF BLOCK CONTAINS SYMBOLS OR SECTION INFO~~TION
THEN **GENERATE SYMBOL BLOCK**
END
OUTPUT-BLOCK [oJ "%"
OUTPUT-BLOCK [3J
=
"3"OUTPUT-BLOCK [6J = s
OUTPUT-BLOCK [7 .. 6+sJ = s-CHARACTER SECTION NAME
P = 7+s **p POINTS TO NEXT AVAILABLE SPOT IN OUTPUT-BLOCK**
IF SECTION NOT ALREADY DEFINED
THEN IF BASE ADDRESS AND LENGTH ARE AVAILABLE THEN **GENERATE SECTION DEFINITION FIELD**
END
OUTPUT-BLOCK [PJ = "0"
OUTPUT-BLOCK [p+lJ = n
OUTPUT-BLOCK [p+2 .. p+l+nJ = n-DIGIT BASE ADDRESS P = p+n+2
OUTPUT - BLOCK [p J = n
OUTPUT-BLOCK [p+l .. p+nJ = n-DIGIT SECTION LENGTH P = p+n+l
MARK SECTION AS DEFINED ELSE MARK SECTION AS UNDEFINED
**GENERATE SYMBOL DEFINITION FIELDS**
FOR EACH SYMBOL
END
OUTPUT-BLOCK [PJ = SYMBOL TYPE OUTPUT-BLOCK [p+lJ = s
OUTPUT-BLOCK [p+2 .. p+l+sJ = s-CHARACTER SYMBOL P = p+s+2
OUTPUT-BLOCK [PJ = n
OUTPUT-BLOCK [p+l .. p+nJ = n-DIGIT VALUE p=p+n+l
OUTPUT-BLOCK [1 .. 2J = p-l **2-DIGIT BLOCK LENGTH**
OUTPUT-BLOCK [4 .. 5J = 2-DIGIT CHECKSUM WRITE OUTPUT-BLOCK TO FILE
FOR EACH UNDEFINED SECTION
GENERATE SYMBOL BLOCK WITH SECTION DEFINITION FIELD GENERATE TEKHEX TERMINATION BLOCK
EXIT
Fig. 4-10. Algorithm for Extended Tekhex conversion program. (part 2 of 2)
Formatted Transfers
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