Subject: Serial SRQ line on SX64 and game cart crashes
            latest updates and corrections: 7-16-2018

     The 6 pin DIN serial ports of our 8 bit computers all have an
/SRQ line on pin 1. It's used for fast serial communication between 
the C128 and a 1571/81, and to the cassette port read line. 
     When repairing several SX computers, I used a game cart called
ROBOTRON 2084 to do a quick check of the expansion port. In two of 
those SX computers, I would get a partial screen freeze when running 
that cart. Doing a drive reset would clear it momentarily, but it 
would continue to lock up when trying to run the game. 
     I searched my notes for anything similar and found something 
called the "Q-link fix for fast loader problems", which was to add a 
pull-up resistor to the SRQ line. That mod fixed my screen freeze
problem with those two SX computers running the Robotron game! I 
installed a 3.3K resistor from P12 pin 4 (the keyboard connector
header) which is a +5VDC source, and P11 pin 1, the SRQ line on the
internal serial port connector header on the I/O board. Those two 
points are only an inch or so apart, so the resistor installation 
was easy. I just soldered it to the back of the board at the header 
connections.
     Experience has taught me that it's not good practice to leave 
logic gates unconnected (floating), but that's apparently what 
Commodore did here. /SRQ goes directly to the /FLAG input of CIA1
UB3. Note that the C64C has a pull-up resistor on its SRQ line as 
does the C128 and DCR models. So do drives such as the 1571 and 1581. 
The 1541 and SX drives do not but the 1541-II does. I must assume 
that the marginal operation of the SX I encountered was just a fault 
of the ambiguous logic state in those computers. 
     I am reminded of the problem of using a datassette -and- a 1581
at the same time on a C64. Someone wanted to transfer files from 
their datassette to their 1581, but it wouldn't work and they thought
there was something wrong with their drive because accessing the drive 
locked up the serial bus with that configuration of hardware. Since 
the C64 doesn't use the SRQ line anyway, the "fix" was to disconnect 
that line either at the computer serial port or in the cable to the 
1581 so it can't pull the line LOW.

Ray Carlsen