Sinclair ZX Spectrum

Chapter 6
51
I/O ports
The RS232 socket is software driven using I/O ports 239 and 247. The function of
each bit of these ports is shown in the following diagrams. Note that although many of
the bits are marked as ‘not used’ by the RS232 port, they are used on a ZX Interface 1
to control the Network and Microdrives. If a custom serial driver is written in machine
code then it is important to set these ‘not used’ bits as indicated to ensure that the
routines will also operate successfully with the ZX Interface 1.
Output port 239 ($EF = xxx01xxx):
7
6
5
4
3
2
1
0
Comms_Out (1=RS232 output mode)
Not used by RS232 (set to 1)
Not used by RS232 (set to 1)
Not used by RS232 (set to 1)
CTS (1=Signal to device ‘OK to send’)
Not used by RS232 (set to 1)
Not used by RS232 (set to 1)
Not used by RS232 (set to 1)
Input port 239 ($EF = xxx01xxx):
7
6
5
4
3
2
1
0
Not used by RS232
Not used by RS232
Not used by RS232
DTR (1=Device reports ‘OK to send’)
Not used by RS232
Not used by RS232
Not used by RS232
Not used by RS232