1.4 COM Port Circuit Definitions

Several common serial circuits will be referenced in this specification. These serial circuits are described in a number of public standards (see 1.4) and implemented in several types of physical ports. These are defined here with respect to the following common serial ports:

EIA/TIA-232-E (current version of EIA RS-232, DB25 connector)
EIA/TIA-574 (common "IBM AT" style, DB9 serial port)
CCITT V.24 (equivalent of EIA/TIA-232-E)

Since these interfaces were originally developed for modems, the names reflect communications. DTE = Data Terminal Equipment (e.g. PC); DCE = Data Circuit-terminating Equipment (e.g. modem).

Table 1 - Referenced Standard Serial Port Circuits

name	V.24	232E	574	source	Description
COM	102	7	5	both	signal common (ground)
TXD	103	2	3	PC	transmit serial data
RXD	104	3	2	device	received serial data
RTS or RFR	105 or 133	4	7	PC	Request to Send or Ready to Receive
CTS	106	5	8	device	Clear to Send
DSR	107	6	6	device	DCE ready (e.g. COM device)
DTR	108/2	20	4	PC	DTE ready (e.g. PC or terminal)
RLSD	109	8	1	device	Carrier detect
RI	125	22	9	device	Ring Indication