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Serial interface of a C64 The serial bus ( IEC or CBM bus) is the standard interface for connecting external devices as. Technically, it is a commodore-specific variant of the parallel -Bus, therefore it is often called IEC bus or CBM bus.
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This interface variant is not compatible with, which is the standard for the serial interface for IBM-compatible PCs. Since serial ports are usually used only for the connection of two devices (e.g. RS232) the IEC bus is a serial. More than only one device can be connected, therefore most of the peripheral devices have loop-through connectors. Theoretically there are up to 31 devices possible ( 0 to 31, 31 is reserved for UNTALK/UNLISTEN).
However, the operating system of the C64 can use only the device numbers 4 to 30, because 0-3 are already used internally. Practically the upper limit is at around 8 devices. The reason for this is, that the (inverting driver 7406) in the C64 can handle a maximum load current of 40 mA. And any device, which is connected to the serial bus draws 5mA.
The selection, with which device data will be exchanged, will be done with the device number. This number between 4 and 30 must be selected on the device and must be unique.
Standard for printers is 4, for the first floppy drive 8. The timing of the bus is specified quite tightly. So if a is connected with an appropriate cable (see ) to a floppy drive and is transmitting data - or vice versa (i.e.
The PC emulates a floppy for a C64) everything may work fine. But if you use a multitasking OS (Windows, Linux.) it will not work without using a complex circuit with own intelligence because of the maximum allowed latenca of only a few micro seconds (especially when using a. For this reason software as and are running under DOS only. In standard operation (, unmodified -routines) the serial bus is capable to transmit 400 byte/second using a and approx. 650 bytes/second using an. With and a 1541 approx.
2.4 kbyte/second are transmitted while JiffyDOS and a SD2IEC achieve 8.6 kbyte/second. The theoretic maximum is at about 20-25 kbyte/second. Pin assignment Pin Description Signal direction Remark 1 SERIAL SRQ IN Serial Service Request In, at the 'Fast Serial Clock' 2 GND - Ground, signal ground (0V) 3 SERIAL ATN OUT Attention, for the selection of a device at beginning/end of a transmission 4 SERIAL CLK IN/OUT Clock (for data transmission) 5 SERIAL DATA IN/OUT Data 6 SERIAL RESET OUT(/IN) Reset, in older unused Signal description. Attention: a lot of fast loaders use these signals differently!
The here described pin assignment is valid only for the standard usage of the bus. All signals (except from GND) are LOW-active.
If they are inactive they are on a voltage level of about 5V (HIGH). Electrically, all signals are, to keep the bus behaviour. SRQ: Serial Service Request in Any device can draw this signal to LOW, to request data exchange with the computer. The C64 does not use this signal, but it is connected with the read line of the (although this can only detect signal edges, but not signal levels). The uses this line as 'Fast Serial Clock' for the / disk drive. ATN: Serial Attention out If this signal is LOW and a is sent, all connected devices on the bus are interpreting this as a command.
As soon as the C64 draws ATN to LOW, all other devices are waiting for the transmission of the address (device number). The addressed device has to answer within a defined time frame, else the C64 assumes, that this device is not present and gives an error message. After the the will be sent, ATN stays LOW. Before the first 'normal' byte it will go back to HIGH state. CLK: Serial Clock in/out This signal is the clock of the transmitted data. The clock will be generated by the device which sends the data.
Linux Serial Port Example
At the rising edge of CLK, the receiver takes over the actual DATA bit. DATA: Serial Data in/out The data will be transferred via this line sequentially by bit. A byte transmission starts with bit 0 and ends with bit 7. RESET: Serial Reset This generates a at the connected devices. Early models of the C64 allowed to use it as input to reset the computer (Reset-button), at later models this is not possible any more.
Therefore a reset-button switch should be connected to the. Expansions. It is possible to increase the number of physically connected devices on the bus. To do this in some of the devices the 1kΩ resistors must be removed. Attention: If they will be removed in all devices, the bus will not work any more.
Php Serial Port
Voltage for low consumption devices. Links. including notes on the serial bus., in-depth details to the IEC-protocol, 2008, compiled by J. Derogee., (short) article from the Compute!
Linux Serial Port Commands
July 1983 by Jim Butterfield., Networking via the serial bus Footnotes.