Hardware manual

ports as "modem port", "printer port", and/or "GeoPort".
Mac also provided conventional RS-232 but at only at 5 volts (which is still legal RS-232). To make it work
like at RS-232 one must use a special cable which (signal) grounds RxD+ (one side of a balanced pair) and
use RxD- as the receive pin. While TxD- is used as the transmit pin, for some reason TxD+ should not be
grounded. See Macintosh Communications FAQ. However, due to the fact that Macs (and upgrades for them)
cost more than PC's, they are not widely as host computers for Linux.
21.3 EIA-485
This is like EIA-422 (balanced = differential). It is half-duplex. It's not just point-to-point but is like ethernet
or the USB since all devices (nodes) on it share the same "bus". It may be used for a multidrop LAN (up to 32
nodes or more). Unfortunately, Linux currently doesn't support this and you can only use it under Linux only
for point-to-point where it behaves like RS-232. So read further only if you are curious about how its features
would work if only Linux supported them.
Since many nodes share the same twisted pair, there's a need to use the electrical tri-state mode. Thus, besides
the 0 and 1 binary states, there is also an open circuit state to permit other nodes to use the twisted pair line.
Instead of a transmitter keeping a 1-state voltage on the line during line idle, the line is open circuited and all
nodes just listen (receive mode).
The most common architecture is master/slave. The master polls the slaves to see if they have anything to
send. A slave can only transmit just after it's been polled. But EIA-485 is just an electrical specification and
doesn't specify any protocol for the master/slave interaction. In fact, it doesn't even specify that there must be
a master and slaves. So various protocols have been used. Based on a discussion of 485 on the linux-serial
mailing list in March 2003, it seems likely that none of these master/slave protocols are currently supported
by Linux.
There is an alternative implementation where two pair of wires are used for sending data. One pair is only for
the Master to send to the Slaves. Since no one transmits on this line except the master, there is no need for it
to be tri-state. Thus the Master may just be RS-232 but the slaves must still be EIA-485. See
http://www.hw.cz/english/docs/rs485/rs485.html for more details.
21.4 EIA-530
EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s (balanced) was intended to be a
replacement for RS-232 but few have been installed. It uses the same 25-pin connector as RS-232.
21.5 EIA-612/613
The High Speed Serial Interface ( HSSI = EIA-612/613) uses a 50-pin connector and goes up to about 50
Mbits/s but the distance is limited to only several meters. For Linux there are PCI cards supporting HSSI. The
companies that sell the cards often provide (or point you to) a Linux driver. A howto or the like is needed for
this topic.
21.6 The Universal Serial Bus (USB)
The Universal Serial Bus (USB) is being built into PCI chips. Newer PC's have them. It was originally 12
Mbps but is now 480 Mbps over a twisted pair with a 4-pin connector (2 wires are power supply). It also is
Serial HOWTO
21.2 EIA-422-A (balanced) and EIA-423-A (unbalanced) 76