SSR-RACK24
24-Channel Solid-State Relay (OPTO22, Gordos) Mounting & Interface Rack

Functional description

Solid-state I/O modules

The SSR-RACK24 is a mounting and interface rack for use
with industry-standard size solid state I/O modules. Thesemodules provide 1500 V of isolation from harsh electricalenvironments for both digital inputs and outputs. Onboard
TTL-level drivers provide the necessary drive current tocontrol any compatible 5 V solid-state relay.

The SSR-RACK24 has 24 mounting positions for solid state
I/O modules manufactured by Gordos, OPTO22, Grayhill,Western Digital, and others. These modules provide 1500 V ofisolation from harsh electrical environments for both digital
inputs and outputs. Mounting screw threads are provided for
you to easily install the SSR modules. An example of an SSR
module and mounting screw is shown below.

Screw terminal wiring
Each SSR-RACK24 module has two independent screwterminals. Use 12-22 AWG wire to connect signals to thescrew terminals. The status LED is ON when a module isactive.
Contact MCC for compatible modules from the SSR-OAC,
SSR-ODC, SSR-IAC and SSR-IDC module series.

Module input/output control
Set the module type for input or output using the DIP switch
for each port.
The screw terminal/module numbers correspond to 8255 ports:

1 to 8 correspond to FIRSTPORTA Bit 0 to Bit 7
9 to 16 correspond to FIRSTPORTB Bit 0 to Bit 7
17 to 20 correspond to FIRSTPORTC Low Bit 0 to 3
21 to 24 correspond to FIRSTPORTC High bits 4 to 7

A – FIRSTPORTA control
B – FIRSTPORTB control
CH – FIRSTPORTC High (controls four bits)
CL – FIRSTPORTC Low (controls four bits)Molex connector

No Connect
PC Ground
PC GroundPC +5V

The Molex connector connects to the PC power expansion
plug inside the PC.

DS SSR-RACK24.doc
Document Revision 4, March, 2010

Copyright © Measurement Computing Corporation

SSR-RACK24 Datasheet

Page 237-pin connector

Onboard buffers

The SSR-RACK24 has one 37-pin D type connector (P14).
Use a C37FF-x cable to connect with 24-bit DIO boards, suchas the USB-DIO24/37.

Because most manufacturers of SSR racks do not supply
output buffers on the SSR rack, simple digital I/O boards suchas the PCI-DIO24, PCI-DIO96, and other manufacturer's
82C55-based digital I/O boards do not have the power toswitch the SSRs. For example, to use an OPTO22 PB16 rack,
you need to use a high-drive DIO board such as the PCIDIO24H or PCI-DIO96H.
In order to be usable with all common TTL output boards, theSSR-RACK24 is designed with onboard buffers. With thesebuffers, you can plug directly into the SSR-RACK24 fromyour PCI-DIO24, PCI-DIO96, or any other manufacturer's
82C55-based digital I/O board.
SSR-RACK inverting logic
On an input SSR, the presence of a voltage raises the TTL
output of the SSR from TTL low to TTL high. Because theSSR-RACK uses the conventional inverting logic, thecompleted circuit of SSR and SSR-RACK lowers the signal tothe DIO board from TTL high (+5) to TTL low (GND). An
output SSR completes a circuit when the DIO board TTL
signal to the SSR is low. The circuit through the SSR is open
when the signal from the DIO board is high.

Do not make connections to pins marked "NC".

Converting the SSR-RACK from inverting logic50-pin connectors

The SSR-RACK follows the convention set by OPTO22 andfollowed by virtually all SSR rack manufacturers — the senseand control logic for the relays is inverted. This means that a 0
output from the digital I/O board causes an output relay toactivate (complete the circuit), while a 1 (TTL high) causes
the relay to deactivate. The chips which invert the logic aresocketed and can be easily replaced with chips that do notinvert the logic. Discuss your order with a technical sales
engineer if you need non-inverting logic.

The SSR-RACK24 has two 50-pin connectors labeled IN(P15) and OUT (P16). Use a C50FF-x cable to connect with
48-bit DIO boards, such as the PCI-DIO48.
Connect a cable from a DIO device to the IN connector.
Connect a cable from another SSR-RACK24 to the OUT
connector.

*324703B-01*