joels@tektronix.UUCP (05/24/84)
Following is an excerpt from an article by Compuserve CBM SIG SYSOP Deepak
Midha. It contains some insites into the C64 1200 baud problem.
Joel Swank
Tektronix, Beaverton OR
Here are the instructions for modifying VIDTEX to work
at 1200. This patch is only TEMPORARY until CompuServe
issues supported patch. This patch will allow you to use
1200 Bits Per Second(BPS) with a modem. It supports all
VIDTEX functions except UPLOADING unless your modem
allows for overspeed terminal devices at 1220 bps. Before we
get into details an explanation of VIDTEX is nesscessary.
Pages 350-358 of the Programmer's Reference Manual describe
some of the things that relate to this problem. The code
used for communication is called United States of America
Standard Code for Information Interchange (USASCII usually
abbreviated to ASCII). This is usually sent in bytes
preceded by a start bit and followed by a stop bit - making
a 10 bit character. ASCII is defined as a 7 bit code but a
byte is 8 bits. The 8'th bit is used for parity to check the
character. The KERNAL can be used to generate parity but
VIDTEX set's the 64 to 8 bit - No parity generated/checked
and sets and checks it's own. This is because VIDTEX uses
bit 8 during B protocol transfer to carry data (This may
cause some problems with Smart modems like the Signalman
Mark XII) and uses the parity bit to distinguish between
text and graphics instructions in graphics mode. The 64
uses a two character field to determine the speed of the
RS232 interface and a formula to calculate the two
characters is given on page 350 of the Prog. Ref. Manual.
The KERNAL contains defaults for 75, 110, 134.5, 150, 300,
600, 1200 and 2400 BPS. Modems which operate at speeds of
300 BPS can handle variations in speed of about 50 BPS but
the new breed of CMOS 1200 BPS modems are much more
sensitive to speed. They also follow their specifications
more closely than earlier models. The Bell 212A spec.
defines the rate between the modem and the Data Terminal
Equipment (The 64 in this case) at a nominal 1220 bps and
the rate between the DTE and modem as between 1182 and 1212
bps. In other words the two speeds are different! The 64,
on the other hand, can only send and receive at the same
speed within certain tolerance limits. The tolerance
limits do not seem to be sufficient to handle the new 212A
type modems. Therefore, the speed of the RS232 interface
needs to be raised to be able to cope with higher rate from
the modem I have tried to use 1212 and 1220 and 1220 works
best with my modems(the Mark XII and a Vadic 3451) but you
might be able to use 1212 which is easier on the 64.
Deepak Midha
73465,777