Section 2 - Commodore Computers

11A 114 COMPUTE! Mov,W82.lssue May, 1982. Issue 24
counting for one error in a transfer of 10,240
bytes, the transfer time would be about six minutes.
I If f the records were 256 bytes long, the transfer
time would be about 5.9 minutes, and with 1024
byte records it would be about 6.5 minutes. It
would seem that the 256 byte format would be the
best choice, but another factor must be taken into
account: the error detection method used.
In the method used for the CP/M system, it is
very simple, and the more bytes it is required to
check, the greater the chance is that it will miss an
error. r. Because of this, the 128 byte format is a
better choice even though there is a small increase
in m the transfer time. Ifa a beller better error detection
method were to be used, it would probably be
beller better to use the 256 byte record format.
A Transfer Format
The actual structure re of the record that is transfen'ed
ferred varies from system to system as well. In [n fact,
it is even less standardized than most other pans parts of
the data transfer format.
Since there is no' real standard forr the record
format, I will describe one of the more heavily
used formats. This s format got its start on CP/M·
CP/M-
based systems and originally appeared in a program
written by Ward Christensen called, appropriately
enough, "MODEM." The first problem when
dealingg with CP/M is its refusal to acknowledge the
existence of a modem. So a transfer program must
provide its own link to the modem.
To begin the transfer, the receiving computer
sends an ASCII NAK K (15H) signal every couple of
seconds until unlilthe sending computer sends an ASCII
ACK (06H). This is the synchronization part of the
transfer. TheT original modem program assumed
that the program was predefined at both ends, so,
once synchronization ni was achieved, the data was
immediately sent.
The record format that is used consists of a
header, the data, and finally y a checksum character
for error detection. The header consists of an
ASCII SOH character (OIH) (0 I followed by the current
record number (starting with number one) o which is an eight bit value. That is followed owed by the
same number, but inverted. (That is, if record 01H0 I H
is being sent, then the second number sent will be
FEH.) This is then followed by the data itself for
the next 128 bytes. Finally, one more character is
sent which is the checksum.
The checksum is an eight bit value e that is the
sum m (without carry) of alla the data bytes sent. The
sending computer compuLer then waits for the receiving recei\' in g
computer to acknowledge that it received the e data. daLa.
The l ~ h e receiving g computer compares cOlllpares its own calculated
checksum against the one that Lhat the one that Lhat
the sending computer sent and, if they match, it
sends an ASCII ACK character (06H).
If they
don't match, it sends an ASCII N NAKA K character
((15H), indicating that it il didn't receive the data
correcLl y. IfLhesending compute r receives a NA K.
correctly. If the sending computer receives a NAK,
it will send the record again. . I If, r. afLer after Len ten tries it is
unable to send the record it gives up and aborts the
unable to send the record it gives up and aborts the
transfer. After all off the recordss have been sent, a
final ASC II EOT (04 H) is sent indicating that the
transmissio n is completed.
final ASCII EOT (04H) is sent indicating that the
transmission is completed.
Some Problems
There are several problems wi th the format that is
There are several problems with the format that is
used and some of the later versions attempted to
correct for this. Unfortunately, this created a new
problem since any change in the basic format meant
it was incompatible with the old format. This tended
to lead towards a real mess with patches to allow
for compatibility to the old programs. Discounting
the versions which were simply adaptations for
the versions wh ich were simply adaptations for
different modems, some of the differences that
to specify it. T here was also change from the check­
different modems, some of the differences that
have occurred are the addition of the program
identifier so that the sending computer cann tell the
receiving computer the progl'am program name instead of
requiring the operator r at the receiving compuLer
computer
to specify it. There was also change from the check
sum format to a CRC format. The identifier has
been implemented several ways, but bUL the most
popular version is also one of the sLrangesL
strangest
i implementations.
III Lations.
A After I' LeI' synchronization has been achieved, , the
currently popular program (MODEM7) (MODEM?) sends the
filename e nanle a character at a titlle time. . That is, it sends a
character and then wails waits
for an acknowledge
(ASCII
ACK) from the receiving g computer. compuLer. Then
it iL sends the next character of the filename and
repeats this until the entire filename has been sent.
After AfLer that, thaL, it iL waits for the receiving computer to
send the checksum of the filename and then Lhen compares
the received checksum against againsL its iLs own inter­
nally calculated checksum. If they are equal, the
nall y calculaLed checksum. If Lhey are equal, the
sending computer se nds an ASC I [ ACK character.
sending computer sends an ASCII ACK character.
The sending computer puLer goes back and waits wailS for
resynchronization. (Waiting (' Na iLing for i'or an ASCII NAK
character.) Ler.) At AL last, lasL, it iL starts sLarts receiving g data normally
y
after the resynchronization ni zalion is achieved.
If there was a checksum error, the sending
If there was a checksum error, the sending
computer sends a bad name character characLer which, for
no particular panicular reason, , was defined as an ASCII // /I
(75H) 1-1 ) and goes back to LU allow resynchronization
ynchronizaLion
and retransmission rCLranslllission of the name. If, I f. after ten len tries
the name cannot not be sent, the transfer is aborted.
Improved Error Detection
Another AnoLher problem that Lhat some versions have corrected correcLed
for fa r is that thaL the checksum method meLhod of error detection deLection
is not t the e most accurate means ofo f detecting an
error. A CRC (cyclic ic redundancy check) is a far