RS-232 Port

MIRE provides an RS-232 port that is functionally compatible to the Radio Shack add-on card for the original Expansion Interface. Refer to documentation for the Radio Shack RS-232 card for operational details.

There are a few differences from the original implementation:

• MIRE includes a 64-byte receive FIFO to help prevent lost data. Vintage Model I software should never require this feature since such programs had to be designed to read the received data quickly enough to avoid overruns. However with this feature new software can afford to spend more time processing received data, and then read all newly-received bytes before continuing.
  
  
• MIRE does not have a "COMM/TERM" slide switch. This switch simply swapped the RS-232 receive and transmit data signals in the original implementation; it did nothing to reverse any of the control signals and would have caused electrical contention if ever placed in the "COMM" position. MIRE is Terminal RS-232 equipment only; if you connect it to another piece of Terminal equipment you will need a null modem or a null modem cable.
  
  
• MIRE does not have an 8-position DIP switch ("sense switches") that can be read by the CPU from I/O port 0xE9. The original purpose of this feature is unclear, but it seems to have been meant to provide default values for various settings, which software could (and generally did) simply ignore.
  
  MIRE includes a new I/O port, 0xE6, that can be written with a value that will be read back from I/O port 0xE9. The default is 7-bit data, parity enabled, one stop bit, and 300 baud.
  
  
• Reading I/O port 0xE6 returns the transmit and receive baud rate settings.
  
  
• Reading I/O port 0xE7 returns the value written to the UART control register (0xEA).

The new I/O registers are summarized below:

	Port 0xE6 (WO): Sense Switch Control
		 Bit  7  - Sense switch 7 (even parity enable)
		 Bit  6  - Sense switch 6 (word length select 1)
		 Bit  5  - Sense switch 5 (word length select 2)
		 Bit  4  - Sense switch 4 (stop bit select)
		 Bit  3  - Sense switch 3 (parity inhibit)
		 Bit  2  - Sense switch 2 (baud rate 3)
		 Bit  1  - Sense switch 1 (baud rate 1)
		 Bit  0  - Sense switch 0 (baud rate 2)

	Port 0xE6 (RO): Baud Rate Settings
		Bits 7:4 - Transmit baud rate
		Bits 3:0 - Receive baud rate
	   
	Port 0xE7 (RO): UART Control Settings
		 Bit  7  - Even parity enable
		 Bit  6  - Word length select 1
		 Bit  5  - Word length select 2
		 Bit  4  - Stop bit select
		 Bit  3  - Parity inhibit
		 Bit  2  - Break
		 Bit  1  - DTR*
		 Bit  0  - RTS*
		 
	Ports 0xE8-0xEB: Per Radio Shack implementation
	
	*Some Radio Shack documentation has these bits incorrectly reversed in its description of I/O port 0xEAH.
	

MIRE uses a standard male DE-9 connector for RS-232. There is also a PCB site for an optional 10-pin box header. Be careful if you decide to use this, since there are two common pinouts for RS-232 headers. The pinout used by MIRE is called "AT/Everex" and duplicates the pin numbers on the DE-9 connector: pin 1 on each is the same signal, so is pin 2, etc. There is fairly common on PC motherboards that provide for RS-232 ports on a separate bracket. Here is the pinout:

DE-9 Pin	Header Pin	RS-232 Signal
1	1	DCD - Data Carrier Detect
2	2	RXD - Receive Data
3	3	TXD - Transmit Data
4	4	DTR - Data Terminal Ready
5	5	GND - System Ground
6	6	DSR - Data Set Ready
7	7	RTS - Request to Send
8	8	CTS - Clear to Send
9	9	RI - Ring Indicator
-	10	No Connection

The other common pinout is called "DTK/Intel" and the pin numbers are jumbled to make hookup easier. For example a simple crimped-on ribbon cable DE-9 connector uses this. It is incompatible with MIRE.

Back to the main page