Is RS-232 or RS-485 data better for access control systems?
A: RS-232 is unbalanced, single-ended and carries both synchronous and asynchronous serial comms between 2 hardware devices at a rate of up to 115Kbps for distances of about 16m. RS-232 needs 3 wires as well as handshake lines for asynchronous connections. Handshake lines are only required if a receiver needs to hold up transmission. Most RS232 asynchronous systems work with only 3 wires. Modems and printers are common exceptions because they are often much slower than the things spitting data at them.
The disadvantages of this standard are that there’s little noise immunity, slow transmission rate over short distances and because it’s single-ended, RS-232 can run only between 2 devices. RS-232 needs a voltage of +/- 25V so it usually draws power from the device you’re installing. Many access panels employ RS-232 as a multi-purpose comms port – it’s simple and in short runs in secure, non-industrial environments is likely to give good service.
Meanwhile RS-485 is a rugged, multi-drop serial data path that allows multiple devices with individual addresses to share a single comms pipe. The technology is balanced and gives full duplex operation with 4 wires or half duplex with 2 wires. RS-485 loops will carry 32 transmitters and 32 – it can be many more than this depending on transmitter and receiver technologies. While 32 unit loads is correct, many modern receivers operate at 1/8th of a unit load or less. Receivers can be up to 1500m apart without signal degradation, with data rate peaking at 10Mpbs. The absolute maximum distance is 1200m at up to 100kbps, reducing to 12m at 10Mbps. Normally Other advantages include the ability to ignore ground noise.
Thanks to Alf Katz, Inner Range, for his technical input.
